there is a post on this subject you might want to look up, less axel to wheel area gives you less bearing surface which gives you more friction in a small area. I tend to agree with dd4h that if all things were perfect no grooves are the best.since the wheels are not perfect the grooved axels give you a better chance of finding a flat area to run on. I'm thinking that's why you will find every set up likes a little bit different wheel gap.I have been experimenting with both types of axels some wheels seem to like the none grooved others seem to like the grooves. TRY both.
hope this helps G.O.