I used to wonder about the same thing: I couldn't keep a good grip with the foreskin rolled back, and if I didn't roll it back it felt like I was stretching the skin instead of the ligs. Nowadays I do both: some straight down- and up stretching with the skin covering the head, but most other angles with the skin rolled back. I feel this is beneficial in both areas (you need more skin to accomodate your increased size- plus the skin actually adds to the size too, at least visually).
Here's what I recommend you try out:
get a pair of rubber/latex dishwashing gloves and use a REVERSE OK grip. I can't explain why it works better (particularily for the lower angled stretches); you have to try it out to see for yourself. Also try and keep just enough blood in the penis that it isn't quite semi-erect, but rather ''full'' and hanging nicely. You can also try doing a couple of quick dry jelqs in between sets to get more blood to the head resulting in a better, firmer grip.
