The 415 chain that comes with nearly every kit just didn't cut it for me. Out of the box the 415 chain had a twist to it that caused it to ride weird on the entering side of the engine sprocket. I rode the bike like that and didn't have any trouble but it just looked odd. The chain seemed to run smooth, as smooth as I would expect with a 2 cycle engine but after the first twenty miles I replaced the chain with Morse-Warner 41 chain, pre oiled it by submerging it in light oil and installed it, with a skate board wheel tensioner. I was amazed at the difference. The noise level dropped dramatically and the chain runs smooth and steady at any speed. The 415 used to jump some.
As for the need for a tensioner; the only reason they would be necessary is to give you some adjustment after the chain begins to wear. If you have a bike with long slots in the rear dropout and can take up the slack as wear lengthens the chain, good for you. If you have little or no adjustment at the rear wheel then the tensioner is a good idea. It might take a few miles but roller chain does wear and it will need to be tightened. Another thought about the tensioner you might consider and most guys will agree with me is the need to secure it to your chain stay by some additional means other than the clamping method. I drilled completely through the tensioner bracket and bike frame ran a grade 8, 8-32 allen head screw with a lock nut to keep the bracket from wrapping into the spokes. (don't laugh, it happened in the first fifty feet, and I had the bracket tight) I don't like plastic against steel either but the polyureathane wheels used on skate boards are some tough little puppies and mine has shown no signs of wear after sixty five miles. It runs quiet and smooth and I'll continue to use it until I come up with a better idea.
Good luck and easy ridin'
Tom