Well it's up to your setup. I almost needed a tensioner with my over rear but I just wonked the chain tight. I do not believe it'll stretch enought o fling off, I hope it does stretch alittle like most say. Otherwise it's just a nice tight chain like I'd do on some bikes and such but the motor sprockets teeth clank the chain going in reverse.
Try it out and see if you like it. I've read on here tensioner on the derailer side, which I took as those bikes which have adjustment on how the wheel is mounted so you can pull it back by screws or whatever. Use that to tighten engines chain, and use tensioner on the other but I don't see why it'd be needed on the other side >_>
If where your motor sits, and chain sized fits alil snug, I've read here people say it should stretch alil and be fine, or need slight tensioning or pull back wheel.
I read here for weeks, you don't really make progress reading and chatting. Test things out and ask questions. I just got mine going 2 days ago, if the tensioner less chain pops off or brakes I'll let ya know