I went down the path of tapping into various power sources as I added stuff. Given, tapping in to the dimmer switch is a must for gauge lighting.
I found that the more accessories I added, the more junked up the wiring became. Eventually I cried uncle and got the Painless Wiring Kit from Geno's. I now take power directly off one of the batteries, thru a fuse, into the cab via the rubber boot above the clutch pedal, and to the Painless fuse block. This fuse block has both switched and un-switched circuits. I mounted the fuse block to the swing down section of the dash that is under the steering wheel. That gives easy access and makes for a neat installation.
You may want to consider future accessories into your wiring plans. Mine kind of grew out of control - gauges, shut-down timer, trailer camera, CB, GPS, satellite radio, etc...
If I trade my truck sometime in the future, I'm going to go with the Painless kit immediately. Having the accessories wired safely from the start will give great peace of mind.