i never take work for a percentage for more than 1 reason.

one is that several attorneys have told me that if you take a percentage, you are more likely to be prosecutable if the site has legal troubles - even if they have nothing directly to do with you.

clients are never obligated to implement your suggestions or use your work. i have one client spend over a hundred grand on his sites and the things he was doing wrong were all pretty obvious but he wouldn't change them. in fact, he had to sell those sites and his program to someone who is now running them at a profit. i'm glad i got paid my hourly rate because i wouldn't have made any money on a percentage deal.

of course, one can take only the jobs where the clients are obligated to use one more as a project manager, but that's really not the same as consulting although i have done both,