If you an afford it, its a good idea. First, cats can be sensitive to anethesia anyways, and if there is some underlying problem that you don't know about yet, a blood test could save your kitty's.
Second, its a good idea to have a baseline for levels so that if your baby gets sick sometime in the future, they have something to compare blood tests too. For instance the doctors discovered I had high platelets a few years ago, but we didn't know if it was because I was sick, or because my platelets were always like that. So its nice to have something to compare things to in the future.
Having said that, most of the time, everything goes fine. So if it were a choice between spaying with the blood test and not spaying at all, or you otherwise can't afford it, it would probably be okay to not have the test done. But it is a very very good idea to do it if you can afford to.