Any unknown dog should be treated with caution. If you're not comfortable reading dog behavior I would suggest staying away all together. All dogs can bite.
If you haven't seen the dogs again they probably went back home. If you haven't seen them before the chance is they simply escaped from their yard.
As for Pit Bulls, they do not deserve the reputation the currently have been saddled with. Any dog can be made mean through mistreatment. Pits which have been properly raised naturally are outgoing and have generally clown like personalities. (Remember Petey from the Little Rascals? He was a Pit Bull and rather typical of the breed's personality.) They were considered the "All American Dog" until 20 or so years ago. Unfortunately they have been the popular breed with the crowd that will typically make a dog mean for whatever reason. A selection of other larger, stronger and less common breeds are not taking their place (Fila Brasileiro, Dogo Canario, Dogo Argentino, etc.) This just follows the historical pattern, first is was German Shepards, then Dobermans, then Rottweilers, then Pits, now moving to the other breeds.
Part of the problem goes to a study published in the Journal of the American Medical Association (JAMA) a number of years ago. The authors looked exclusively at reported deaths resulting from dog bites. Over a 10 year period there were just over 250 instances over the entire US. Considering the number of dogs in this country this is actually not a statistically representative number, but the authors drew conclusions about dangerous breeds based on these numbers. Pit Bulls came up first. The AKC and AVMA (American Veterinary Medical Association) among other organizations took issue with their conclusions and statistical methods and the authors re-did the study shifting the evaluation dates forward by 5 years (still a 10 year period.) Several issues were found including irregularities in breed identification. (Often news reports claim a dog is a Pit Bull even if it is something entirely different. As an example, we had a serious attack, though not fatal, here a couple of years ago that the paper reported quite extensively as a "Pit Bull Attack". The dog was a pure bread Great Dane. This was never corrected in the paper.) The breeds which the revised study identified as the most dangerous was a completely different set, and included Jack Russel Terriers. The authors recanted their original claim of dangerous breeds and agreed with the AKC and AVMA that behavior of each individual animal and not the breed should determine the classification of "dangerous". Unfortunately, certian insurance companies and local governments have continued to use the original (invalidated) study in black listing certian breeds. You can tell if they're using the original list if "Husky" is on the list of banned breeds. By the way, over the last 6 years the Rottweiler is the breed that has accounted for the most deaths though the number is still extremely small as compared to the total number of dogs. (This does not in any way discount or try to downplay the impact of even a single death from a dog attack. Each is a devistating event.) There are no accurate statistics regarding dog bites in general or serious attacks specifically. Claims about the number of attacks are usually heavily tilted by the agenda of those reporting the numbers, either very high or very low.
The AKC, AVMA, HSUS, AHA and most other national and state humane and veterinary organizations oppose the use of breed as a criteria in dangerous dog legislation. Pennsylvania was the first state to actually make it illegal to include breed as a criteria for this determination. Prince George's County Maryland has one of the most severe anti-Pit Bull laws and has been extensively lambasted by these organizations with even the county animal control director stating that nearly all dogs seized under the law were sweet and friendly family pets. Several attempts have been made to pass a Pennsylvania type law to prevent breed descrimination but it has never passed (though it came quite close two years ago.)
I've seen some wonderful Pits come through our shelter, and some very nasty dogs of all breeds (the bad ones are clearly a very small percentage of the whole.) While I've seen some extremely dangerous dogs come through, the meanest, nastiest dog I have ever seen was a Toy Poodle. Given half a chance this little dog would have ripped your hand off. While a dog this size may not present the same potential threat as a larger one it still goes to show that the issue is the individual dog (and really reflects on those that raised it) and not the breed.
Finally, the claims that Pits have such great bite strength or can lock their jaws are simply not true. They really don't have a bite that is greater than any other similar sized dog, and the greatest bite strength ever measured in a domestic dog was in a German Shepard (Wolves, by the way, have a much greater bite strength.) All Terriers tend to grab and shake when they attack, and Pits are Terriers (American Pit Bull Terrier!) This does not mean their jaws lock, just that they tend to behave in this fashion. This was bred into them since Terriers were originally bred to go after vermin or small game. They were supposed to go after the prey, latch on, and hold and shake until the hunter or handler caught up or dug them out to finish off the prey.