From a safety perspective, absolutely cap on the day it goes void. It’s something I’ve insisted on since coming into post. My background as a gas engineer has given me the experience that when an empty property gets broken into the first thing that tends to go is the copper pipework. If the thieves don’t have the sense to turn off a live gas supply then the repercussions could be catastrophic.
At least with it capped, if there were a break in and the gas copper pipe was taken then the risk of a leak would be mitigated.