When it's okay to be off-meta in Ranked:

1. When you're a team of friends and you really don't give a hoot.
2. When the champion you're picking synergizes extremely well with the rest of the team. (So that would mean you're last pick or in a Team.)
3. When the champion you're picking counters your lane opponent extremely well. (Assuming you aren't first-pick)

EDIT: 4. When the champion you're picking is so stupidly overpowered that they can't lose lane, and didn't get banned.

When it's not okay to be off-meta in ranked:

1. When you're first pick. You have no excuse to be doing that, since you're not counter-picking. It forces the rest of your team to pick around you, or lose because your pick forced the team into lacking something it needed.

Granted, there is an exception if your team knows exactly what you're doing and can play champions to pick around you. That requires either very fast typing to communicate, or that you're a ranked Team. It's also only going to be good if it forces your opponent into a counter-pick that the rest of your team will stomp on.

2. When it doesn't synergize with the rest of your team.

3. When you saw it work in LCS. You are not in the LCS. You are not fighting the same opponents with the same champions as in that one match. Just because one of the very best players in the world can go Teemo jungle with the right teammates, the right ally champions, and the highest amount of communication possible doesn't mean you can do it well in Bronze III SoloQueue.

EDIT: 5000000000000000000000 (However many 0's are in his name) noted that it's also true you shouldn't be doing a META pick if it doesn't fit your team.

However, picking Fiora top is perfectly fine as first pick, because, while it does give your team less in terms of their picks, she can fight well against almost any top-lane pick. Meanwhile, Karthus top as first pick (who should instead just call mid) takes a lot more away from the team.