Words of wisdom...
Don't play play money games, don't play freeroll MTTs, don't play micro cash games. In general, avoid any games where the way villains play is so far away from real poker that you end up learning bad habits because they work to exploit the ultra-fishes you meet at those stakes.
In online poker, you usually don't have time to think as much as you would like about each hand, putting villain on a range of hands and assessing the EV of each line you could take against this range. Especially when you multitable, which is a must if you want to make serious money. So you end up making a lot of decisions in a sort of Pavlovian way. You take decisions because you've been in similar situations a lot and learnt that a given line gives best results on average. Learning bad habits at micro stakes is something that will make you lose a ton of money once you move up.
In that sense, i think micro players should focus on STTs until they have the bankroll to play 10nl at least. Because the correct strategy for STTs remains the same whatever the stakes you play - play tightish early, push/fold appropriately when blinds go up - so you're not picking up bad habits by playing them.
