I've played in 3.0, 3.5 and 4.0 USTA leagues. Our team usually plays two levels at once -- say 3.5 & 4.0. We put out our strongest pairings at our rating level USTA leagues, and then tinker with partnerships when we play up. We also use Interclub (off-season) matches to practice partnerships as well.
As mentioned before, complimenting strengths is a sure bet -- strong baseliner with strong net, or consistent player with power player.
Also -- I'm from a smaller club. For us it's essential to have 2 or 3 partners we can play with. And also a minimum of 4 singles players for each squad. With mom duties, match conflicts, illness, injuries, vacations, kids' commitments, husband business trips -- it's essential to have back-ups.
With a 10 match season -- we've found a 15 member roster is about as low as you can go -- and that's with a captain who is willing to scramble up players last-minute. Again, most of our players are 40+ years old with families /or jobs.