There is no way that your profile or Job Success Score can be harmed by withdrawing a proposal.
On the other hand, you don't really benefit from withdrawing a proposal, either.
Keep in mind: If the client has hired somebody but needs another person, or if the original hire doesn't work out, the client may want to hire one of the other candidates.
The most successful freelancers here in the forum tend to use a "fire and forget" method with proposals, rather than fretting over their submitted proposal list. Like Preston said, there's no negative effect other than that if the client wants to reconsider you're no longer there in the list and may miss out on the job. But there's also no benefit to withdrawing them either - you don't get the connects back, for example.