For Klingons, like the Vikings of Ancient Earth, the best afterlife is reserved for the great warriors. Their religion and mythos hold that a warrior who dies valiantly in battle, facing a worthy foe, will almost certainly be granted a place in Stovokor.
So when a Klingon says, "Today is a good day to die," what he means is "This is a worthy foe. Should I fight him and fail, it is not dishonorable." Honor is a bit important to Klingons in the same way that water is a tad damp. Faced with the decision to flee and live or fight and maybe (or even certainly) die (but perhaps protect others) a Klingon warrior's duty is to fight and die.
When, on the other hand, fighting and dying will not save anyone, or when it is not needed (such as because a relief force has just arrived, who can easily handle the threat) it is NOT honorable to die in battle. It shows you are too cowardly to stand tall despite your wounds or defeat.
Thus, when Worf sees no option but to ram a Borg ship, hoping to destroy them and save Earth and his fellow Starfleet officers, Worf decided that today was a good day to die - there would be much honor, and something eminently worthwhile would be accomplished. When the Enterprise appeared, fresh and capable of turning the tide, such an act became unnecessary. The day was no longer a good day to die - it would have been dishonorable.