If your talking about I.E. Stainless Steel sheet counter tops etc. you should let it cool and warp then peen it after its cold until the warp is out. Buy doing it this way your controling how much to peen until the warp is gone.  If you over peen you will stretch the material and warp it the other way and you will have to re-heat to shrink it again. It will always shrink until its cold. You can't control how much peening is needed when its hot or even warm.  Type 316L  is the worst for shrinking & warping. If your doing butt weld's on 16ga. S/S sheet counter tops, peen evenly back and fourth across the full length of the weld with a steel dolly until the warp is out. Any hammering after it's flat for taking out dent's for grinding etc., a OAK WOOD DOLLY should be used, it will prevent the metal from stretching. A piece of OAK 4" x 4" x 8" long, is the size of dolly I've used for years. I drilled a 2" hole 4" deep in the bottom 4"x4" end and filled it with lead. Think of pinching a piece of clay between your fingers, it gets thinner and stretches out just like the metal will do when using a steel dolly. Remember the "LENGTH" of the weld shrinks.   After it cools the welded area is shorter in "LENGTH" than it is an inch away from the weld in length on both sides so something has got to give, and this is what causes the buckle and warp. It will shrink evenly across the of the WIDTH of the weld all the way so nothing is getting shorter or smaller than any other part.   This is why you should "NOT STITCH WELD" or use "CHILL BARS", This makes it worse, waists time and doesn't prevent the length of weld from shrinking. Anytime you melt (weld) S/S it's going to shrink and no amount of chill bars or stitch welding is going to prevent it or lessen it. This is an old wives tale, and sadly a lot of people sware by it,. . . . Just take a minute and think about it!!!   Like anything, it takes trial, error & technique, DON'T get stuck in one place, pay attention to everything that happens when you "weld here and peen there",  and just keep what I said above in mind. After over 40 years in the Custom Sheet Metal Trade I could write a book on this subject.