I think most of us play a mech until you lose twice in a row then switch it around. I play mostly my Atlas but swicth out to my Stalker or Victor on accasion to play a different style. Sadly i also switch to my Jenner and forget it cant go toe to toe with an assault mech
I start out with whatever machine I'm tweaking or leveling, and if that doesn't work for me I change it up. Unless I'm in a bad mood, then I go for one of my shit-kickers or dakka builds. I usually end up here at some point during the night, especially if I'm trying to tweak a build.