RE: front wheels.. It might depend on the quality of the independant.. As an example, look at the FLY Classics. At least some of them have two stub axles in the front. They are 1/8" The axles are plastic and the wheels are push on.. If there is no obstruction betweem the two stubs you can remove them and put in a full width steel axle. I have found the single axle to be considerbly faster.
The stock stub axles has a lot of play and wobble, if the bushing were wider, and more substantial ( brass maybe) . These stubb axles get worse with time. I have never used nail polish or zero traction tires, but i imagine that it would increase the single axle's advantage even more.

I think the issue with a solid axle is the two wheels trying rotate a different length which scrubs speed. But the stopwatch is the final say......

My opinion. I peefer front wheels to not touch the track/setup block. Just a hair between tire and track. Consider an oval track like Harry's. When you enter a corner the car leans to the right. Since wheels either don't touch or just barely touch, the lean pulls the inside wheel clear of the track so it doesn't touch anyway. Only the outside wheel touches the track and rolls.