I say it's a matter of the type of wheelin and terrain you want to cross. Even if you only go once a year, if your in nasty terrain, the locker will help.
I have a locker in my Land Cruiser and I wouldn't do without it. I only went wheeling 3 times this year and I'm glad I had it every time. But the terrain warranted it.
You do have to realize that having a locker will make a vehicle handle rather unexectedly in certain situations. Joey eluded to this when he mentioned side hills and such. If you understand how the vehicle will react in those situations, your line of attack and style will be different than if you have no locker at all.
How do you learn this???? Practice dude, Practice.
I wheeled for 4 or 5 years without a locker. Every time I went out I became a better driver without having to modify my vehicle. As my confidence and experience grew, I challenged tougher terrain. Finally, I found the limitations of my vehicle and I went and got a locker and then it was a whole new ball game. I've been wheeling with a rear locker for 6 years now and I'm to the point where I want to put a locker in the front now.
Somedays I find wheelin too easy, and I wish I had an unmodified rig so that it would be challengin again. I guess thats why I bought the Disco! (Hee, Hee).