By "SO cards" I assume you're referring to the Good and Evil Objective cards. Revealing your objective gives you an extra rune, but doing so at the end of the game only helps you if you have the most runes.
Say you have 6 runes and you have completed your objective. You wait until the end of the last turn to reveal that you have completed your objective and gain the 7th rune. Fine. If another player has 8 runes, you still lose.
Besides which, waiting to reveal the objective also leaves open the possibility that the condition will be undone. For example, an enemy might attack your capital and force one of the two developments to be removed. Now you have to rebuild the development before you can claim your objective, whereas if you had claimed it immediately, you would have gotten the rune right away. If you wait until the end of the last turn, a tactical strike by an opponent in the last year might ruin your objective without giving you enough time to rebuild.
Road to Victory allows you to "call it", whether you have the most runes or not. As long as you have at least 6, and can hold them for one year, it doesn't matter if your opponent has 7 runes or 20. You win, one year later, if you still have your 6 runes. It allows you to win "early," cutting short your opponent's time frame, but you do have to fight for it. Nothing is free, after all.
As to whether it's wise or not, that all depends on how well fortified your runes are. =P