I would NEVER use a trickle charger, but that's another story. In the old days I would always take the caps of when on the charger, and then put them back on when it was fully charged. Charge them up and then let them sit another month off season before charging again.
Recently I had a problem with a battery that was still under warranty. The dealer said NO NO NO don't take the caps off when charging. Just for laughs I asked several other dealers. They all said the same thing.
Now if you insist on trickling, and thus keeping the acid boiling all the time, then maybe you should check weekly for fluid loss, whether you use the caps or not.
