David said exactly what I was going to (pretty much:) but put it more concisely and probably better. Like he said, I too have not encountered a warp problem that couldn't be fixed with hot water and patience. What I've done in the past is to boil a big enough pot of water, remove it from the heat, let it sit for several minutes and then just "trial & error" it while paying really close attention to A-pillars, window straps, or other thin, fragile areas. Another thing David mentioned that I think REALLY helps is the slight over-correcting and quenching with cold water after your final tweak.
Being a little ocd, I generally chill the water with ice that I use to "shock" it with:) but I doubt that's necessary!
Also, using a sink has the advantage of being able to direct the hot water from the spigot to a particular trouble spot in addition to submerging it. An interesting thing I've found is that in my case the hottest tap water will easily straighten a Polar Lights Ford/Merc body but I have to use the simmered boiled water to straighten the Revellogram early-mid '80s Monte Carlos, Buicks, etc.
John