I am trying to keep my house cool and avoid cooling some rooms that I don't use. I have closed the registers in the room, but should I close the doors? Or does that cause a negative pressure situation. Should I seal the registers instead of just closing them?

Thanks
Adam

Sure, close the doors.
Ideally you would have dampers in the ducts. If you never want AC to some rooms, close off the ducts at the source. I think you can buy vent covers, or you can cover them with cardboard and duct tape, but all that cold air is still working it's way along the ducts so it's only going to be some help.

I would close registers and close doors. If the door is open you will be coolinmg from the rest of the house.
I wouldn't seal real good as a little ac may help reduce humidity, mold?

I would not close off any register to any room. Your system is set up to move X amount of cfm if you start shuting down registers and can not move X amount of air you will have a higher amp draw on the blower and in turn will have a higher amp draw on the compressor. That's the reason why we add bypass dampers to zoning systems. Shutting off registers can actually cost you more $.

Most new installs I see a return vent to allow flow and not be under pressure.

When you let heat build up happen inside the home. Not cooling rooms on the interior side of your home with out insulation in the interior walls-- that will be a heat gain on the system. Shut off 1 room to save you a dollar may cost you $2.

I agree with T-Top on this one.