I'm not a pro at this by any means, but this is what I did. I have a BCD396XT and I programmed just one P25 site and then one group, using the red/blue control frequencies for the repeaters closest to me, Paxton, Onyx and Homestead. Once that was done, I just leave it to search and pull in the talkgroups. Once I consistently see the TGs that service my area, I will lock out the unnecessary ones that I don't want to hear. I can also just put in the TGs I want and set the function to scan, and then it just scans those I choose. With this P25 system, I'm pulling in plenty of traffic from other Divisions, that I used not to hear on the original Trunking system due to them being too far away.
Right now, my South Desert Division 4 is being dispatched on multiple TGs, originally 3535 (3-CMD9) Fire Division 4, but now I'm hearing most of our dispatches on 3521 (3-DISP2) Desert/River Dispatch and 3507, (2-CMD5), which is Division 3 Fire response, up in the Mountains. Due to this I'm keeping 3535, 3521 and 3507 currently scanning. 3537 is our Division's tac channel, 3-TAC9, which is active for us. Each division should have it's own Fire tac. Someone told me LPTAC9 is a County EMS Tac, but I'm not sure if it's conventional or digital, so I can't help you much there. I do hear it mentioned frequently though, same as you.
I'm sure others can give you much more technical details about programming, but I've been happy with what I'm getting on the new system, and am only interested in pulling in the traffic directly related to my area and job. I hope that gives you a starting point at least!