Didz says:
Present tense when I'm describing something that is happening in the game.
e.g. Moli mousey-sneaks down the corridor, careful to avoid treading in the squeaky floorboards in her path.
Past Tense when describing something that has happened.
e.g.
The coach journey to Bogenhafen was completed without incident, which was unexpected.
As a rough rule of thumb I use 'Present Tense' to narrate events that the players can influence, or react to, and past tense to narrate events that are finished and cannot be altered by player action.
Very, very much this. I use present tense as I perceive everything to be dynamic and can change based on other people's posts and the roll of the dice. Using past tense feels like something is already locked in, as if it's already happened and can't be changed. If, as a GM, one of my players uses past tense, it really throws me off, even if what they describe is somewhat open ended (i.e.
"the dwarf swung his axe aiming at the goblin" as opposed to
"the dwarf sliced the goblin with a swing of his axe" in a situation where you're still making the attack roll and don't know if the dice will agree. Statement of intent, yes, declaration of outcome, no). I'll rarely use the past tense when describing things that have transpired and most of these times it's in a summary post at the start of the chapter (i.e.
"it has been a few months since X happened and the party has been busy researching Y and Z in order to understand it, when just this morning they received news of...")