Part of setting up your handlebars is adjusting the handlebars in the supports, back/forward, rolling them etc.
Reasons to do that is to have a comfortable standing position, with a nice reach to the bars and good control of the bike's steering. Kids that grow for instance can use some adjusting of the handlebars forward/back, even if they don't complain about it - evaluate their position every now and then.
Most competition dirt bikes offer a few settings. KTMs for example generally offer 4 different positions forward/back, with almost an inch total range, so about a quarter of an inch (6 mm) for each setting. Check your motorcycle's manual - if you don't have one, try to find a copy online or order one.
It sounds very small to worry about but I can asure you, each makes a big difference in how the bike handles, because of the leverage added/removed due to changing the place your input force is applied versus the rotating axis of the front forks.
You should start where you feel most comfortable in the standing position and try at least one position forward and one position back from there, to see which suits you best. They will change the bike's handling, but do give each enough time to get used to it.
Don't forget that this forward/back position of the handlebars is combined with rolling the bars in their supports - basically you can finely tune each position by rolling the bars forward back in their supports.
If you need more adjustment than possible from factory, you can get aftermarket handlebar risers, which not only allow you to rise the bars, but also adjust them forward/back as well, such as the Roxx pivoting risers.
NOTE: make sure to torque the bolts involved precisely to what the manufacturer recomends. These bolts are very important and you need a calibrated torque-limited wrench. If you make them too loose, the handlebars may roll or even come off impacting the safety of your ride, while if you go too tight, you could strip the bolts and also cause damage to the forks/clamps or bars in a crash.
NOTE: after adjusting the handlebars, always check the cables - move the handlebars from steering lock to steering lock and make sure the cables are not too tight in either position. Also, the engine rpm should not change (meaning the throttle cables are too tight). If the cables are not fine, you need to either adjust the bars or the controls to have the cables operate properly.
Other dirt bike setup topics:
Here's some other related readings and videos: