My personal feeling (and what I've always done) is that if Mom lets or wants you to touch them, go right ahead. I've always pet them gently on the head, their little bellies, talked to them in a low voice. You could even nudge their little heads gently over a nipple if at times they seem to be having trouble finding them.
In fact, sometimes when I'd rub their little bellies they'd grab my finger with their tiny paws, and I'd wiggle my finger a little, and maybe gently touch their nose so they would get used to my scent. As long as they're not crying or afraid.
Actually, the early contact with people makes them more trusting and people oriented as they grow, I think.
What I personally would recommend though is not to pick them up unless necessary (like checking them or cleaning the box), not to take them away from Mom (I always waited to clean the box when she was eating, and then put them right outside it, and then right back in). I also suggest being carefuil of loud noises and bright lights, at least until their eyes open.
I know some people like to take them out and take pictures, but I always would just take a few while they were nursing and their faces were away from the camera or any strong light, and would only take a few. Once their eyes were open, that's when I felt safer being able to take them out, play with them, hold them, take pictures, etc.
Just my opinion, and my experience. Don't you just want to love them like crazy?