At the start, I used to put points on (for example) if a Striker.."Attack & Physical"....he needs good attacking skills, but also the physical prowess to cope, not get too tired in match's etc.
As I progressed through levels, playing higher quality teams, I realised there comes a time when you need Attackers to assist in defending & Defenders to help out in attack...so started levelling them up equally.
Also Nordeus have been clever in the distribution of skills, so that a GK needs def skills and Physical skills (obvious) but also, in the "Attack" section are "Passing & Crossing"..equally as important for a Goalie or Defender in RL, also, in the "Defence" skill set, there are "Positioning & Heading" (very important for an attacker).
So my thoughts are, you need to level up equally, or you dont get the best out of any position.