I'm with Static on this one. I remove the vent liner just about every time I use my flinters. The only exception is when I only fire a handful of shots at the range (it's so cold here sometimes that I give out before the rifle does). Otherwise, I unscrew it and make its cleaning part of my regular cleanup. However, when I put it back in, I apply a good dose of choke tube lube, so it won't seize the next time I try to remove it. I also use choke tube lube on the threads of my caplock nipples and in-line breech plugs, and I find they all come out without too much difficulty. Also, it's not necessary to torque these vents, nipples, or breech plugs down for them to stay in place. A good tight (but not overly so) fit is all you need.
Oh yeah, one more thing. If I've shot a particular ML a fair bit over the course of a year, I'll replace its vent liner, nipple, or (in-line) breech plug (as the case may be). I do it even if an examination reveals no obvious signs of wear and tear. Probably an excess of caution on my part, but I'd rather be safe than sorry.
:m2c: