Troll is probably the best person to ask.
However, it depends on your list and what you're vs.
I always mulligan for my aspirants/wild growth/innervate in control match ups. That seems obvious, but you'd be surprised. Typically if I get one of those, more so the former 2, I'll keep a 4 drop like shredder or savage combatant (a tech choice of mine, not required in any Druid deck) when available.
When playing vs aggro or paladin, I mulligan for wrath/swipe/wild growth/living roots. Keeping the board clear before turn 6 is one of the only hopes of winning since Druid removal is dreadful.
Vs warlock, it's a toss up to mulligan for bgh or swipe, and whether or not to keep either. With most warlocks now being a variation on of renolock, both cards can be helpful for early game board. Also nice to keep keeper of the grove for twilight drake.
It's not a perfect guide, but hopefully it's a start. I wrote this in a couple minutes while taking a poop at work. Gl m8