Panning keyboards

Not sure what you mean by 'effect' I guess you might be referring to string 'sound' or piano 'sound'. Firstly are your keyboards hardware or software? Are they in stereo ie two outputs. I would start by recording any synth or keyboard parts in stereo. You will find many have nice stereo imaging already.
 
Panning them in the mix means you can either keep them hard L and hard R but also with a plugin like Cakewalk Channel Tools you have the option to narrow the images down as well.

I usually pan strings to the left and piano to the right.
 
Strings are usually stereo, so I use the Channel Tools plugin to shift them rather than the pan slider. Piano may or may not be stereo - mono for dense mixes where the piano is not a featured instrument, stereo for when the piano's a major component. In the latter case, it's Channel Tools to shift the piano to the right. This places the left-hand portion of the piano in the center and high notes that convey melody to the right where they'll be able to cut through better.
 
(For purely orchestral stuff, I prefer to pan violins, violas, cellos and basses in the traditional way: first violins hard left, then second violins and piano at 50%L, violas center, cellos and basses 50-100%R. It's an odd arrangement by pop/rock standards, having your main melody hard left and bass hard right, but it's how they've been doing it since the 19th century so who am I to question it?)