Any time you need to go round, go round.
In theory you should be landing in a configuration which will allow you to go around if need be, even if only at a sluggish rate.
There is nothing to stop you going around even if the wheels have hit the ground, i've done so before.
If i were you i would judge each situation on its merits. If you can land safely, do so, if not go around. Weigh the concequences of each option and make a decision.
