Joey,
For the most part, each TG is locked to a specific cell. Most of the fire TG's are on Countywide. The benefit to programming the other cells is that a couple of TG from each zone are on the lower-level cells:
2-M/2-N - North Cell (2-N is the major incident tac assigned for structure fires and such)
3-A thru 3-E and 3-J/3-K (I think) - South Cell
4-I/4-J (not postitive if it's these 2 or not) - Nothwest Cell
5-G/5-H Laguna FD - Laguna Cell, 5-H simulcast on South Cell
5-I/5-J (same as above, not sure) South Cell
Sorry that I'm not positive on these, but I don't have the info available at the moment
So bottom line, You need to program in all cells (Except Sotuhwest) to get all the TG's, but 90% of the fire stuff is on the Countywide Cell. If you want all of the ALS hospital contact freqs, that's a whole different story.......
Mark