So far i've been distributing points equally across all my stats because i wasn't sure what classes i wanted to level
At what point would it be better to place points according to my prefered class?
and was it a mistake placing them across the board?
It honestly depends although if you're playing Melee classes it might be better to focus on STR, VIT and DEX, and if you're playing caster classes it might be better to focus on MND, INT/PIE, VIT. That being said, the game is pretty forgiving about stat allocations and you can supplement a 'balance' build with stat conversion traits to swing it towards a Melee or Caster class. That, or you can re-allot your stat points (but that's a pain when you want to rank up multiple classes)
I don't know when it would be best to place points according to your preferred class, although I think the sooner the better. For ref, I started speccing specifically around r18-20. Just keep in mind there is a stat cap per rank so you don't need to get 90 STR on an r20 Marauder and gimp yourself in other areas. If you can, it might be best to just hit stat cap on your primary stats before dumping the rest in whatever you want.