Usually by the way boss fights go are a good indicator that you need to increase your str. If you take more than a certain 'time' to kill a boss, this 'time' is specified by you. For example when I started the game (not as a swordsman but an archer) I could kill the early bosses in 2-5 hits. I set this up as my condition. As the lv of the bosses increase it became harder to finish them so I invested in str till I could finish them off in same amount of hits as the previous bosses.