Unless it's lop-sided, I would leave it alone, because it will attain a certain height and then 'catch' up, like children do. However, if it is necessar, after leaf fall it can be pruned. Phil J

Nipping out the tips will encourage branching. Is your acer a bonsai or a regular tree & what height is it? As to when to prune I do this in the Spring just before the new buds open. Trompenburg is a superb acer, any chance of a photo Japon?
Also I agree with Philjeffs re leaving it to develope. I have actually done this with one of my acers & it has gone from a two branch leggy stick affair to a really nice two tier little tree with no pruning what-so-ever. Have a look at the recent photo's I have just put up for you to see ( the one with two green arrows pointing it out). Good luck.