I think it is much more preferable to allow the OP to vote on answers, than not to let them.
Blocking voting until an answer is accepted wouldn't contribute much to making people accept answers more often: the majority of people who are not accepting answers often enough are probably not voting anyway.
From the definition of a vote as you hover over it, "This answer is useful" or "This answer is not useful". Considering the OP (let's say Alice) is the one who asked the question, she's the one who knows best whether the answer was useful or not to her situation. Upvotes are also useful to reward people who may not have been the direct solution to the problem, but did provide a useful answer.
Consider this question of mine. OrbMan was one of the first people to post, and I upvoted his answer because it was helpful in teaching me new things as well as making me think about how to store things. But I had no intention of accepting his answer unless I truly intended to implement it, should I have been barred from rewarding his presentation of knowledge just because I hadn't accepted an answer? This is especially the case if you have a difficult question that later on is assigned a bounty. This means you definitely don't have an answer yet that you can accept... but does that mean that the current answers are necessarily unhelpful? Would their helpfulness really change just because someone else posted the answer that was accepted? I eventually accepted Jeffrey's later answer because it directly answered my question, but I still found OrbMan's answer to have been helpful until then. He deserves the upvote as a reward.
The OP is just as much a member of the community as anyone else. She should be free to judge whether any or all of the answers to her question are helpful or not, and contribute her opinion in the form of votes.