What he may be referring to is that the string vibrates less wide (is that a real phrase?) if you play further up the neck, which means you may get less buzz I think. The trade-off is you also get less sustain for exactly the same reason. In all honesty though, I have played with fantastic bassist and some very average bassists and never noticed a big tonal difference when they were amplified. On an acoustic bass things show up a little more readily.

Mike is 100% right - Play where you feel most comfortable. If you're first starting out I would worry more about getting locked in the groove while singing.

Also, don't worry about "looking" like a loser playing bass and singing - no musician worth his salt is going to say anyone singing and playing bass is a looser if they're playing in the pocket and singing well. Work on the show AFTER the sound.

Hope this helps!!

Best of luck!!

John