Tali and Garrus can be dead in ME3, right? And if Garrus is alive for ME3, he can be ignored for the whole game except that cutscene after the genophage mission, right? Tali is only required for 2 of the 4 missions, dreadnought and Rannoch, right? Outside of those 2 missions, Tali can be ignored, right?
Yes. You only need to talk to Garrus on the Menae mission, and after the Tuchanka arc finishes (and really, Victus does most of the talking, Garrus just says "it must be hard losing Mordin" and ends it.
Tali is only required to be spoken to at her introduction, during Priority: Geth Dreadnought, and Priority: Rannoch. After that, she can be ignored.
Both of these are related to their race's respective character arcs: Garrus is the one with the pull in the turian military, and Tali for the quarians. Speaking to them makes sense, and you can keep it professional from start to finish (telling Tali you came for the Migrant Fleet, telling Garrus you'll sleep when you're dead). Liara, from the beginning, gets a creepy M!Shep introduction and commandeers the entire XO cabin for her own devices, invites herself to any area of the ship she pleases, all without permission (Seriously, if you're the Shadow Broker, you don't have other ships or hidden bases? How the hell did you get that stuff onto the ship, then?)