I have a Parker 51 that caps a a little roughly. That is, it doesn't glide on an off the pen; there is some resistance. Should I try a little silicone lubricant on a Q-tip inside the cap or are there any other recommendations?

More than likely the fingers that grab the clutch ring have either bent or out of alignment... no amount of lubricant is going to fix that...
Generally this is a repair you don't want to take on yourself because you might actually do more damage and they need a special too to remove them

Has it always capped roughly or did it start recently? Is it scratching up the barrel?

If I get a cap that is slipping off I'll pull the clutch fingers out some, which will result in it needing a bit more force to cap than usual, but it will break in after a bit. It really doesn't bother me much, Id rather that then it slipping around.

Jon, you really don't want silicone grease in the cap. You'll have it all over your fingers when you use the pen. Nasty.

We're so used to cap clutches that are worn or compressed, that it feels strange when a 51 cap clutch is tight. But a clutch that is loose can lead some folks to jam the cap on until the shell sticks on the inner cap. This in turn can lead to a ring pressed into the surface of the shell, or a cracked inner cap, or the nightmare "stuck cap." The clutch, not the inner cap is supposed to hold the cap onto a pen. Some clutches do have burrs that need to be removed though, and their rough edges can lead to a scratchy feel as you put the cap onto the pen.

When I restore a 51, I tighten up the clutch fingers (pull them in) so that they grip more firmly. Then I polish and smooth them with 1500 grit so that they don't scratch the pen. Wrap the 1500 grit paper around an Xacto knife handle. Rotate so that the paper is folded back. Flip over and rotate the other direction.