League of Legends (LoL) Question: Flat runes or scaling runes

Depends on the rune. 9x Greater Seal of Scaling Health is common on solo laners and a lot of junglers. Supports usually run Greater Seal of Health for higher stats early game since they earn less XP and need the lane pressure. ADCs usually run Greater Seal of Armor for similar reasons.

Flat AP glyphs are used on some junglers, but most mid laners run either Scaling CDR, Flat MR or Scaling AP. For junglers and top laners, Scaling MR, Flat MR and Scaling CDR are all common choices.

For just about everyone, marks and quints are always flat stats.

Just remember that solo laners and junglers tend to make better use of scaling runes since they have faster XP gain, while bot laners usually opt for flat stats so they aren't at a disadvantage early, especially since they level up slower.

Flat AP glyphs are used on some junglers, but most mid laners run either Scaling CDR, Flat MR or Scaling AP. For junglers and top laners, Scaling MR, Flat MR and Scaling CDR are all common choices.
so when u put flat runes u get more control

Scaling breaks even with flat somewhere between levels 5 and 9, but it still depends on your playstyle.

Some junglers will run 9x Greater Seal of Armor because the effect is huge for the early physical damage taken in the jungle, but an anti-magic-damage rune page could use 9x Greater Seal of Scaling Armor or 9x Greater Seal of Scaling Health instead.

If you want a safer early game, you'll need the flat runes. And the scaling runes are better for the late game but their effect is way less impactful since you'll have more stats by lategame anyways, so even if the stats are 2x better the overall observed effect tends to be way less. I guess flat is the safest way to go and you should only pick scaling when you know why you're choosing them.

Normally you take Scaling Runes unless you plan on going aggressive early on for kills. In which case you take the flat rune that is most likely to help you against your opponent. So for example if you are top lane then you'll likely want flat armor or health runes depending on your champion and the enemy champion. Health runes helps champions with any health scaling and I believe is more useful early game. However I don't particularly know for sure why to take health over armor, other than with champions that use health scaling I mean. I believe Health is best when you are dueling a champion 1v1, but armor is better when against multiple champions.

For jungling you often take flat armor runes for junglers with less sustain so you take less damage and therefore can help your team more in the early game and recall less. If you plan on doing early ganks on mid lane you might benefit from flat MR runes too however I normally prefer scaling MR regardless.

For mid lane I normally take flat MR runes on Kassadin because I always plan on first blooding my enemy at level 3 or 4 by widdling them down and then flashing all in. However normally scaling MR is best because it quickly helps you a lot more than flat MR runes.