I'm currently wondering about which runes to buy, percent runes or flat runes. I've seen several champion spotlights (by RiotPhreak, I think) that recommend flat runes for several builds (Teemo 
, Kennen 
, Jax 
, etc.), and am currently wondering if percent runes are worth my trouble. What are your thoughts?
Beowwulf1 voteswhy don't you do the math yourself? Just type in google search x% of (champion stat) and compare it to flat.
AzyWng1 votesI was hoping for a recommendation of things like wether you prefer to use flat or percents, which ones are better in which situations, etc.
Beowwulf1 votesThere aren't many runes that do percents. If your talking about health I wouldn't suggest it. 3% of a champion with 4000 max health is 120 health, not that much of a difference to someone who already has a lot. And runes don't even give a health buff that big. (Late game 120 health is 1 attack from adc.
Given that adc's are the only ones that target tanks that gives you about a half second more of life.
If you mean scaling I heard riot should be buffing scaling runes soon where scaling runes are equal at level 6 to flat. In that case I'd always recommend those except for some adc's that start with really low attack damage.
Brian Lenz1 votesPercent runes are going to be good for champions where you actually buy more of that stat. Example being buying % Health on say Mundo. You're going to be building health somewhere, so for every health purchased, you get more out of it.
Flat runes are the opposite. Jax being a good example of this. You might buy health somewhere (Triforce), but it's certainly not a core component of your build. Therefore the flat health is going to be a lot more effective than additional %.
Following that logic, Flats vs Percent are a lot like Flats vs Scaling. Flat runes are going to be more potent in the early game, where the Scaling and Percents are more significant later on.
Best advice though, try both and see how they feel. It'll give you a much better idea of when and how to use each.