For me it depends on where the keyboard is. I generally use my keyboard at the edge of the desk, which is what I was taught in the stone age when I took "keyboarding" class. (Learned to type on a typewriter before new-fangled computers were so popular)
I think I can easily use this position for hours on end b/c my chair has height-adjustable arm rests. I highly suggest investing in a chair like this if you don't have one.
At work, I often see people at who put their keyboards at arms length in front of them, resting their arms on the desk. In this case, you would benefit from a wrist rest most likely.