The only time to change your look for an audition is when you are told to. I held an audition a while ago looking for a Sarah Palin, everyone came dressed in character. One of the best afternoons of my life.
There is a fine line between coming to an audition dressed as a type (no suit to Grease) or a character. If you come dressed like a character you run the risk of clashing with the creative team's vision. I had people dress like cowboys when I held auditions for a show with a old-west-ish theme, but that wasn't my thought at all, so it didn't help them, and some came with accents and everything that hurt their chances. On the other side I held auditions for a Christmas show, and one guy came dressed like an elf, sold it perfectly, and was cast before he was out the door.
David walked into the valley
With a stone clutched in his hand
He was only a boy
But he knew someone must take a stand
There will always be a valley
Always mountains one must scale
There will always be perilous waters
Which someone must sail
-Into the Fire
Scarlet Pimpernel