Stay at 90kts. Simple reason, it makes the maths easy. 90kts is 1.5nm/minute, 100kts is 1.6667Nm/Min - which apart from being infinately harder to work out "at a glance" - isn't enough to make a difference in the great grand scheme of things.
And flying schools - contruary to your belief - don't make a huge ammount of money from you (Unless you're on an integrated course). Margins are very slim...