keepInventory on, almost always. Minecraft's 'difficulty' is the least interesting part of the whole game and, unlike most games, removing that aspect doesn't lead to the game becoming boring. This is especially true with the big modpacks where you're cocking around for hours to make some particular item that lets you travel somewhere or jetpack around or whatever. You die, lose that item, and you then either can't get back to it (because you've softlocked yourself out for hours due to losing the gear you need to get to where you died) or you're faced with just re-treading the same grind to re-acquire the items, which is not interesting or gameplay.
Multiplayer is different.