When you have 100% approval, you get a bonus to population growth (I think it is big, but I forget the number). During expansion phases when I have a number of small-pop worlds, I try to keep their approval at 100%.

If you're not trying to push pop growth on new worlds, there's no reason to keep approval high (other than role-playing tendencies). Scoremonsters tend to run just above 45% until the turn before a Senate election, when they drop taxes enough to stay in power, and then jack them right back up on the next turn.

The time to the next election is displayed in your Civilization Manager under the Government & Ethics tab. The weeks to the next election is counted down each week. You don't actually have to reduce your taxes until the turn that it sayes that elections with be held at "the end of the week" at which point you reduce taxes, end turn, win the election and raise your taxes. It's usually a good idea to save the game prior to an election just in case.

Note that winning an election is based on total (i.e. global) approval whereas the approval on each particular planet is what determines that planets growth rate.

The critical levels of approval are:

20% and below, population declines 10% per turn.
From 21% to 40%, population is stable.
From 41% to 75%, population grows at "base growth" rate. Base growth is 4% to 7% race dependent.
From 76% to 99%, population grows at 1.25 base growth rate.
At 100% approval, population grows at 2 times base growth rate or 8 times base growth rate if Super Breeder.

The above information comes from the Wiki Population article. 

I would love for them to fix that election thing. It would be better if it was looking at the entire year instead of just 1 day. Seems like exploiting the game to me.

But then again on occasion I have been doing it myself so....