A shallow dent where the wood fibers are not broken can often be completely raised as if it was never there. And even dents made by contact with sharp objects such as rocks, where the wood fibers are damaged, can be raised somewhat and much improved in appearance. Naturally, if a chip of wood is missing, steaming isn't going to do a damn thing.
Dent raising is best done during a refinishing process after the old finish has been stripped, because stock finish will prevent water from penetrating into the damaged area. Trying to raise a dent by steaming a polyurethane finished stock will not work as well because those finishes are much more water resistant than older oil and varnish finishes.
I used to use an old steam iron, but found that it worked better to make dedicated dent raising irons of different sizes out of flattened copper rod and copper tubing with wooden handles. You can heat these irons with a stove burner or propane torch. Water soaked pads work well, but I found that regular 70% isopropyl rubbing alcohol works better. It helps a lot to allow the alcohol to soak into the damaged area for a while before applying the hot irons over a damp pad. By doing this, you are generating steam from within the wood rather than trying to drive steam down into the wood. It is the rapid expansion of liquid into steam that raises the dent. The wet pad is necessary to prevent scorching the wood surface. Note that alcohol will dissolve shellac finishes, so greater care must be taken if you are attempting a spot repair without a complete refinish.
As with any home gunsmithing project, I'd suggest practicing on an old broken stock before tackling Grandpa's Parker. You can create more dents in the practice stock to learn what works best.
edit: Forgot to mention that it may take several steamings to get the dent raised as much as possible. As long as progress is being made, keep going. I've hit some stubborn dents 5 or 6 times.
