How mandatory is it for the members of a tribute band to look just like the people they are honoring? I mean physical appearance, not just style. I ask because some people are rather unique. I dare you to try and find a competent bassist that looks like Geddy Lee, or a vocalist who both looks and sounds like Steve Tyler.
On that note, how similar does the vocalist need to sound to the original to be taken seriously? Does he/she need to be precise, or is just being a good singer who nails the material enough?
Well, since you're asking for the ideal, here's my objective take on it:
The front person is the most critical aspect of a tribute band, just as with any band. He/she had better either LOOK a hell of a lot like the original, OR SOUND a hell of a lot like the original, ideally a little of BOTH. If it's one or the other, he/she should SOUND like the original. People will forgive you if you don't look like Steve Perry, but if you SOUND like him, you will knock most people off their stools.
There is a Queen tribute band from England, and the lead singer has got the look and the sound. He's got Freddie's wardrobe, hair, and mustache. He's got Freddie's moves. It's pretty damn impressive if you're a Queen fan.
That's the ideal.
As for the rest, wardrobe can get you down the road, but you sometimes have little control over people's physical features, so they had better try to at least dress the part and they had better nail the music.
I've seen a lot of Beatles tributes with a right-handed Paul who didn't look much like Paul. But he dressed the part, sang very close to Paul, and nailed the basslines, so I was sold.
In short, do your best, do everything in your power. If you have two candidates for a part, and one has the look AND the chops, that's your obvious winner.