If you don't have game time, you can't queue for PvP if your XP is disabled. If you're okay to play at 20, go ahead and unlock it. If not, you'll have to unlock your XP and deal with the XP you get.
Note also that queuing with XP disabled, whether because you have game time while it's locked or because you're a 20 without game time, puts you into a special queue with other people who locked their XP. Most of the time this means that queues never pop and you'll only see PvP if you actively schedule it with other people. F2P 20s and any paid players who lock between 20 and 29 might get some pops because that's the bracket F2Ps go into. Know that randomly queuing can easily put you into games where you get steamrolled, so a lot of F2Ps get bored and quit PvPing unless they actively plan their games too.