true, but useful
If you're one of many companies making presentations, and you don't erase the board using Windex/409/etc, you didn't plan well enough.
I have been in conference rooms where the previous presenter's notes have still been on the board and my first question to the client is 'Can I erase this?' This does several things: 1) gets the previous presenter, (and their flawed logic :) ,completely out of view if not out of their mind, 2) gets me close-up to read what was left (or marginally wiped), 4) provides unquantifiable bonus points in the eyes of the security concious when, at the end of my presentation, I erase my own board.
Note: if you have a choice, and they're all bold colors, use the ones that erase most eficiently. You don't want to spend more time trying to erase process flow changes than you did drawing them.