Here in CT we have* an emissions test program that tests the gas cap.
I've seen nearly new ones fail the test and crappy old ones pass it.
If you have a similar test where you are, you may be forced to replace
yours if it fails. Otherwise, I would only replace it if you have a
problem with leakage or vapor, or if you find that there is a vacuum
when you go to open the tank- that means the vent in it is plugged and
the fuel pump is fighting to move fuel out of the tank. I can't really
confirm that this failure mode ever happens with Volvo caps, because
I've never seen this particular event on a Volvo, but this does happen
with some other cars.

*Note: CT's emissions testing has been suspended until August as they
work up a new testing system, but the gas cap test was a pain in the ass
while it was in effect. Come August, thousands of people will probably
find that they are not in compliance again...
--
Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"