Ok, the way i was taught to repair cracks was to humidify the bejesus out of the guitar to bring the crack back together, and then glue it. I am now of the opinion that this is a BAD way to repair cracks, because the owner will innevitably let the guitar dry out again in the future, and the crack will just end up reopening or moving somewhere else if you did a good job with cleating. Forcing a crack closed with clamps seems like a similarly shortsighted approach.
I currently think that any crack that is not completely closed when the guitar is fairly dry should probably get a splint. This is not always cost effective on cheapies, and it is really hard to do a good job of color/grain matching on any guitar, the tricky thing is that they are all a little different and it is hard to know what is the best fix for a given guitar.
For splints, i usually rout a channel with a laminate trimmer, the take a slightly overtight splint and smoosh it a little bit in a vice, so that it fits in tightly and will expand a tiny bit when waterbased glue is applied. It works best if the splint does not end anywhere- run it from the purfling to under the bridge, or under the pickguard, or fingerboard, etc. The point is dont stop a splint in plain sight or it will never look good.
The back is obviously trickier than the top. If a back has lots of nasty cracks, i would rather take it off, get everything in good shape, and then rebind with slightly wider purflings to make up for any shrinkage. Again, not always cost effective.
If a crack is pretty tight, but not all the way closed when dry, i may widen it very slightly with an x-acto blade, and the glue in a curl from a handplane. If you get lucky that can end up looking pretty good.
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