What is considered the lightest bait weight that a bait caster should cast well?

That's a VERY general question and depends greatly on the individual reel, but there are baitcasters out there that can cast 1/16oz. very well - The Daiwa Presso and Daiwa Pixy come to mind 8-)  

I use an 1/8 oz jerk bait alot and can cast it pretty far and easy with my Sol

 Need to factor in the rod also...

skillet

The reel will illustrate the line test that best fits that reel.

The rod will do the same.

A light bait needs a light line so this will be your key when reviewing the line tests on your rods and reels.

I use light tackle when throwing light baits.  All on a spinning rig.

You will have to experiment to see what works and what doesn't.  

I much prefer baitcasters to spining gear but when it gets to less the 1/8 ounce or a little more BCers are just not worth the extra effort. BC rods also are very important. Don't try light lures on MH or H rods and use light line 10 pounds or preferably lighter.