Me personally, I don't really stretch my sack out a whole lot while shaving.
I do stretch out my unit tho, to get the hair on the sides and such.
When I first thought about shaving with a razor and shaving cream, I was scared I was going to cut myself, especially my sack, but the first time I tried it, it went very smoothly and I actually love using a razor and shaving cream and won't go back to using anything else.
Maybe it go's so smooth for me because I use a 4 bladed razor and the red Barbasol shaving cream which is very thick cream, plus it's dirt cheap.