Yeah I always thought the best place was opposite the valve to help with balance, but to hell with balance it’s a dirt bike, put it within a few inches of the valve or it will be hard to get the tire on. Choose the rimlock well, some types are better than others, make sure they are the right size for the tire or they will do squat. My last rimlock was from the rather dubiously named ‘General Abundance Company’ of Taiwan or wherever. It might work, but I won’t go as far as recommending it.
My foolproof tire fitment method has been learnt from countless road race slicks mounted onto rims too small for the intended purpose. It goes like this:
Take it & some beer to a friend who has a tire machine, I hate knobblies.
Easy huh?