With a sufficiently stiff stock/foreend, my preference is to keep the barrel gap nice and snug:
The picture makes it look just a bit closer than it really is due to the offset of the camera, but hopefully the point is made.
In this case, getting a uniform barrel gap was pretty easy since it was already in the ballpark. I simply laid the barreled action in the stock with the rear action screw not quite tight, placed a strip of sandpaper under the barrel, and started sawing it back and forth (and fore and aft) under the barrel. Obviously, don't do this with a finished barrel or else you'll probably wreck the finish (the back side of the paper gets pretty abrasive from all the debris it collects). The last little bit was done by wrapping some sandpaper around a deep socket of appropriate ID, since I didn't have the correct size dowel.
On stocks with too large of a channel, I wrap the barrel in plumber's tape to generate roughly the desired gap, and then fill it with West System epoxy and their "micro balloon" filler mixed in to create a thick paste. This sands really nicely once it sets. Once upon a time, I decided to do this fill operation using Devcon putty; that did not sand quite so nicely afterwards
