Custom brushes are made by generous people to be used by others, so you might aswell use them if you think they'll benefit your piece. But obviously as many have said before you can hardly say its your own piece if they arent your own, but you can however, say it is your own arrangement - you might not think it but arranging graphics and cropping images is an artform in its own right, it takes quite an artistic eye to make the best possible selection in an image for the use in your piece. The average sig is best considered artistically as a colláge. hence why those who make sigs, and use brushes are generally referred to as designers as apposed to artists, they use the materials available to them to create something that has a function