Re: Present Tense, Past tense?
I've definitely seen a tense be 'mandated', or strongly encouraged as part of the game itself.
I personally don't advocate anyone sticking to a set tense different than they feel they should use for a given post. Even changing tense within a post is fine, as long as it progresses in a logical sort of way (meaning, not changing within a sentence or paragraph :p).
Frequently, a post is a direct response to the present situation and also the immediate past. Your character was in a scene where something happened, but you weren't online while it was happening or (more likely) the post didn't hold itself up waiting for your input. NPCs will say several lines, scenes will proceed a certain amount, without you being "at the table" to interrupt the GM like you might be able to face to face. Especially in a slow posting rate game.
It seems most elegant to me, even if you prefer present-tense, to touch on the immediate past a bit at the start of your post.
[As he listened to the old man's story, John couldn't help but roll his eyes] makes sense in the context that the old man's story just happened.
You can then transition back into the present in following lines when you finally respond. "I've heard it all before," John spits. "What actually happened that night?!"
That reads a lot better to me than trying to revise past events by forcing your entire post to present tense.
I'm much more interested in writing that is interesting to read during the game than presenting a crystal clear narrative to an outside party. If it makes sense to have your post be past tense for a given scene, fine. If it makes sense later on for another to be written in present tense, fine. If you transition in one direction over the course of a somewhat lengthy post, that can work too.
In an action scene, using present tense can help heighten the tension. In more laid-back scenes, sticking with past-tense may be the way to go.
And if you're really tired and you mix up tenses within a paragraph (I imagine we all do it sometimes) that is acceptable every now and again as long as I understand what is happening.