I try to role-play it based on who my Shepard would take with him/her (usually her)
For example, since my Shepard always likes Tali, she goes in the party all the time. Except the one time I played renegade, so she stayed home.
Apart from that, I had one MaleShep who took Tali and Miranda, since he was trying to get into Miranda's pants and wanted more opportunities to talk to her.
One FemShep who took Tali and Garrus, because she was trying desperately to hold onto the past (which made bumping into Liara fun)
And one FemShep who took Tali and whoever would be most suited to the third position based on the mission at hand, because she was a professional and saw her team as tools for a job, and you shouldn't try to force someone into a position they can't work in. (Generally this worked out as Tali, Infiltrator Shepard and then one of Grunt, Jack, Samara or Legion) Tali took engineer spot, Shep took sniper, and the third took supporting fire and biotic support.