Assigning sp points is simple really! , think of how long you intend to keep the player, for me two/three seasons tops, so if he is a goal keeper or defender then keep assigning point to defence and stamina but just keep an eye on attack! , dont let it get to low compared to others!, there is an exception to the rule!, if your player has an SA say ariel then add more points to attack as he will be good in the opposing box too so chip in with a few headed goals.
Midfield and strikers go for attack and stamina!, if you want to be conservative then just select the auto assign option, the down side to this is, your attacking player might get alot of defense etc but it will even out in the long run!