I had a friend ask me today about upgrading his car audio. He wants to get a 5 channel amplifier and use just four channels for now, with plans of adding the subwoofer on the fifth channel later on when he gets more money. I told him this should be no problem at all, and just wanted to double check with you all.
It wouldn't be a problem other than you might limit yourself on what subwoofers you can get. I think the best and most future proof way would be to buy a 4 channel amp and then buy a mono amp when you decide on a subwoofer. That way you can buy the subwoofer you guys want and get a amp that would match up with it. Instead of trying to find a subwoofer that would match with your 5th amp channel which probably isn't going to put out much power.