Real Quick: Does it Matter to Attack First?
According to distinguished datamancer Balco: "Yes... but not much." Unless your name's on Pyke's list!
Distinguished datamancer Balco — with whom you're hopefully well acquainted, and in case you're not, here's an in-depth interview we did with him recently — recently calculated the difference in win rate for the most common Champion combinations, depending if they attack first (in red) or second (in blue):
Main takeaways if you're not playing with/against lurkers:
Most differences are small enough to fall into the "don't sweat it" range,
Some decks (notably Thralls, but also Darkness and Legacy Poros) actually prefer going second,
Riot certainly did a great job in making the coin a small factor (see "don't sweat it" above).
Main takeaway when Pyke and Rek'sai are around:
Lurkers is indeed a highroll-y deck. Depending how the coin goes, Pyke & pals are either the worst Meta deck (and by a long shot at that), or one of the best (nearly matching Ahri Kennen).
For reference, here is Balco's original tweet.
And somebody, please, get that man a Ko-fi! =)