To your question: "At what point does an up vote make a difference. I have noticed that early votes do not but that after several hours, my up vote can increase the amount by several cents."

For upvoting I think there is a bit of a trade off between rewards for the upvoter and rewards for the author. If a minnow like me votes after a post is already worth say $1000, I might add 10 cents to the overall payout, but if not that much more money gets added on after when I vote, let's say even an additional $200, at that point I might not even earn the minimum curation rewards of .001 SBD. If I voted early, for example when the total post value was only $25 and the post ended up being $150, most likely I'd earn a little bit of a curation reward.

Discussion on earning rewards as a curator
Because of your question, I did a little bit more research and I had forgotten about that commenters share in the Author rewards of the post. So as a curator it's best to make a comment as well as upvote at a good time. I'll have to test it again but I think even upvoting your own comment can help boost your curation reward on the post as well. (@johnsmith wrote a good post a while back - https://steemit.com/steemit/@johnsmith/how-i-increased-my-curation-rewards-by-more-than-1000-and-how-you-can-too )

Timing of your vote also matters. If you vote before 30 minutes you are sharing a portion of your curation reward with the author. If I am around I usually do vote before the 30 min mark because I want to try to get ahead of the other people that will be potentially voting and adding more value to the post. Voting early is also a great way for you to give a larger reward to the author. Voting at the 15 min mark is a 50/50 split of your rewards with the author, voting at the 20 min mark is a 75% curator/25% author split. @shenanigator mentioned this in his Steemit FAQ under 'How do reward distributions work?'.

As a manual curator, I regularly use @lantto 's Automatically upvote at x (minute) bookmarklet on chrome to time my votes. Link: http://steemlets.com/

I also am going to look into upvote bots since they can help with the upvoting around or before the 30 minute mark on my favorite authors.

http://steem.li/curators is also an interesting site to watch to see what the best upvote curators are upvoting