Most of the interview strategy books suggest completely shaving your mustache before interviewing because a superbly trimmed 'stache is 'neutral, at best'. In fact, when I showed up for ground school after getting hired, all the clean shaven interviewees that got hired all were growing their mustaches back, including myself.
Remember, the people who ware interviewing you are probably fairly conservative and think "Limp Biskit" is a result of a horrific mistake at the local Cracker Barrel.
Personally, I'd suggest not showing up for a serious interview with a goatee, get hired, and then try to 'feel out' social environment before growing it back.