Thorsim -
Generally, if you're sure you'll make 3 out of 4 frames, do register in advance. Some CO's are even happy with 2 our of 4, just let your Group Leader know well in advance. Further, if you can find a personal replacement, say a squaddie, who is willing to fly that frame(s) you'll be absent, that's even better. Numbers fluctuate a good bit each frame anyway, so it's much better (and more fun) to register and join in the pre-Scenario discussions and strategizing, which you don't get to do as a walkon.