Depends on the laws of your state. WA it is illegal to have a loaded weapon anywhere in the vehicle unless you have a CPL. If you have a CPL, no reason not to keep it on your person 100% of the time if you can. Personally I do not like the idea of the weapon not on my person. If for any reason you get carjacked, or have to get out of your car instantly for any reason you are leaving it behind. I know most officers are going to react strongly to seeing a weapon in the vehicle on a traffic stop, whether you are carrying/store legally or not, as 1bad mentions, chances are high you will have their weapon pointed at you. Out of sight out of mind is #1 Some states also have a duty to inform (not WA) which means you have to tell the officer as soon as he pulls you over so keep that in mind also, even then it is better for you to inform you have a weapon than for them to walk up and see it.
Much will depend on the gun you carry as well as how you carry. I am righty, carry right side and never have a problem with seatbelt, even when I carried a full size. If you like to carry behind the back though, you are kind of hosed, there is just no way to do that comfortably sitting in a vehicle. Now carry the way I do means it is tough to get to, almost impossible while belted in. However I can and do put it on and off sitting in my seat when not belted. But you can use releasing your seatbelt as a distraction to draw your weapon if you are in a carjack situation. They know you have to do that action of reaching for your side to release the belt and your other hand up keeping their attention. Then your left hand can also be used to defend/control thier weapon as you bring yours on target as you step out of the vehicle.
Glovebox can cause issues as the officer asks for Reg and proof of insurance, it is also damn small for anything other than a single stack.