While I agree with Rob's post, I would add that a larger ownership stake brings some additional benefits, such as possibly being the lead investor in a round (which means the ability to negotiate terms), obtaining a board seat (versus observer rights), and having a larger role in planning and negotiating future financing rounds and exits.
Here's a link to his post:
http://robgo.org/2012/07/09/why-do-vcs-have-ownership-targets-and-why-20/