I haven't had much luck with the freezing method even though we have very good freezing capabilities here in alaska. I get the dents out by capping both ends and using a torch, either mapp gas or oxyacetilyn. it is over pretty fast. the only time i have had to use compressed air is when there was a leak in the pipe that i didnt have time to deal with. usually i find them at weld them up at the same time. pressures over 60 lbs are easy to get just by heat alone if you have a decent seal. radiator hoses with pipe plugs clamped in work pretty good for me.
if you do try freezing them out, remember water freezes from the top down. you want the liquid to be right at the point you want to bend back at the very end of the freeze. two stroke pipes that are oily inside (as most all are) can be harder to get dents out of that way. the ice just doesnt get the "traction" it will in a fourstroke and staight walled pipe. It is worth trying if you have a chrome pipe though.