That's a good question. I can make suggestions, but only seat time reading terrain can dictate when a locker is needed. Your driving skill goes a lot further than a locker will in most cases.
I'd say that 90% of the time you should lock it before you think you need it. Stuff like coming across a long section of two track that is heavily rutted and full of water is an example. Scout it first, and if it appears to be doable, go to low range, lock the axle and have a plan if you don't make it. Once stuck, it's unlikely a locker will get you out. It won't hurt anything on dirt to have it locked, but if it's not needed, don't use it.
The '05 Tacoma is like most vehicles these days in that the locker is only available in low range. There is a modification to the system that will allow you to use the locker in hi range too. It involves adding relays and switches to the wiring that controls the locker switch. There are detailed posts on how to do that on the TTORA forum. A rear locker in high range is very usable in sand.
First off, are the tires upgraded to an LT all terrain or mud tire? If not, that's the first thing that really, really needs to change. A P rated tire is, IMO, not even worthy of needing a locker. P rated stock tires are so thin in the sidewall, if you get into stuff that requires a locker, you're probably going to have other problems. With a stock '05 Tacoma (we owned a mildly modified '05 DC/TRD for many years by the way) on a good tire like a Toyo AT 2, Falken Wild Peak, Cooper AT, Toyo M/T, or GoodYear MT/R Kevilar that will not cut a sidewall very easily, you can get into stuff that may require a locker. Good tires also allow you to air down to get better traction, and a softer LT tire is less prone to damage than a P rated - P rated will cut if you look at'm funny. Having a good compressor is a requirement - the MV50 sold at O'Rielly's is a good example of a quality compressor that doesn't break the bank.
More often than not, I use mine in snow and sand. I'll lock up before venturing into deep snow or super soft sand. They also come in quite handy on some of the more technical trails I do.
http://youtu.be/7xy6RI4Pqfc
Mud is a rarity, but we do find it every now and then, and a locker works well in that too.
There are exceptions to locking up an axle. On off camber situations, a locker can cause you to move sideways as much as forward. Again, it all comes down to experience reading terrain.
You should always go with at least one other vehicle unless you are very well prepared like we are. A winch and all it's associated gear, complete recovery gear selection including chains, shovel, hi lift jack, a real jack, a good spare.
Have fun, and Tread Lightly
Mark