Depends on the complexity and the quality level of installation. Some clients ask for the down and dirty least expensive installation possible, I pass on that business. My current clients get the best installation possible and are willing to pay for that work. My primary power is fused using two or three 100amp bolt down "Mega Fuse" housings and associated fuses with the secondary systems getting a fuse block with 4 Maxi Fuses. About 10 years ago I started using STINGER brand Maxi-Fuse blocks and although they are designed for audio systems they have been excellent fuse blocks with solid connections and 100% reliability. As for battery drain I have switched all my clients into systems like the Whelen CenCOm Carbide or Sapphire that have built in timers to shut down the power after the vehicle is turned off.
If I am working on a Police Package vehicle I use some of the OEM power supply cables for emergency equipment, it's there, it's reliable and I cannot recall ever having a problem with the Ford or GM power. I always run cables to the battery for computers, camera systems, license plate readers and other equipment that needs that uninterrupted power.
I never use any wire not specified for automotive use. I purchase wire from "Wire and Cable Your Way" on the internet and always specify the silicone coated automotive rated wire. I quit using butt splices and solderless connectors about 20 years ago, all my installs are soldered.