Can I pour concrete if it's raining out?
If you're manually mixing under cover (garage, gazebo, kitchen fly) and only dumping when ready, then no problem.  

Definitely cover in case of heavier rain.  Its never a bad idea to slow cure concrete, anyway.  Plastic can make flat work look bad (plastic wrinkles), but for deck piers, no problem.

Now get to work and quit stalling by playing on the computer ;-)