I try to be careful not to have too much in any one account. Although 5d is one of the most respected books out there, and I am not worried about anything "happening" I do try and spread the wealth. I'd recommend withdrawing down and opening up on another site as well. Not only does it allow you to shop around for best lines, but it can give you peace of mind should anything happen to a book, or something as simple as a server crash prevents you from betting that game you really want.
Personally, I have funded accounts on 5dimes, bookmaker, heritage, bovada, and with a local. I always know I'm getting the line I want (or at least the best one I can get), and won't get shut out.
As for your question about 10k being "too much" I cannot answer that because I have no idea what your personal financial situation is and whether or not you are comfortable with that on one site. Personally, I would not be, but a buddy of mine has a higher risk tolerance and amuch higher roll and he keeps 10k on a couple different sites. Personall, I try not to have more than 5k with any source at any time.