I have played through the first game eight or nine times now. After getting the achievements, I have always had Ashley as my tank and usually Garrus as my engineer. Wrex if I don't need one. Its nice to have both tanks backing you up.
Ash and Wrex can handle the Survival missions on Pinnacle Station all by themselves. After making the needed time, I figured I'd keep going until the Geth got me. Thirty minutes later, I just sat Shepard down and figured I'd wait until the Geth got my squad (and then me). Thirty minutes after that, I called it.
In ME2, I always take a tank and a support. The tank is usually Garrus or Grunt. The support is usually whoever I think fits best with the character concept I have for that Shepard. The middle characters, like Thane and Jacob, never see any playtime.