I've seen a few people worrying about their voting power, e.g. This post by @verhp11. There's some good information on the Steemit FAQ.
The best analogy I've seen for voting power is that it's like a swimming pool that is continually being filled. It takes about 5 days to fully fill. Each vote you give takes some water out. 10 votes of 100% take a fifth of the water, so it will be full again in a day, if you didn't vote any more.
If you don't vote at all it just stays at 100%. That's a waste as your votes help to distribute the daily rewards.
I've been aiming to get my voting power down to around 80% as it will recover enough each day to stay around that. It means my full vote is worth a little less, but I rarely give a full one anyway. I get this information from the wonderful Steem Now by @penguinpablo. It's good for keeping an eye on what votes you get and your rewards.
It doesn't matter too much if your voting power drops lower, but you will have to wait longer for it to recover. There are times such as Steemfest where I give out a lot more big votes. If you know you will be offline for a few days then run it down and it will be back up on your return. In any case I think manual voting is best as we want the best content to trend. Of course vote buying distorts this, but that's another topic
New Steemians won't have the option to give a partial vote. That comes when you get to around 500 sp, but using busy.org or @steem-plus is supposed to give you the slider. I can't test this as I have so much sp.
I'd advise new users to not worry too much about trying to earn from curation rewards. You need a lot of sp to make that worthwhile. Instead you should vote up posts you really like. Leave good comments that add to the conversation and those can earn you good rewards as well as helping build your followers.
I hope I've got the details right here, but I welcome corrections and questions. It's really worth reading the FAQ first.
Steem on, and vote freely!