Originally Posted by Launger
Dump mana regen for more armor pen, buy manamune for infinite mana.
However champs like kog maw/twitch might benefit from attack speed aswell.
masteries are usually 21-x-x indeed, if you use ghost/teleport then get it from utlity, but oherwise the extra 9 defense points are better.
Also protip: if you get manamune and that point in defense mastery that increases hp regen per mana, you get like 50 hp regen with buffed manamune, which is pretty handy.
And for jax always go 9-21-0 for that arador point.
I would never get Manamune.
From what i've seen, Twitch works great with Critchance
Don't play your carry with teleport, except you got some kind of tactic with it and it fits the team.
Jax is not a carry.
For champion choice @ OP
Ashe offers alot of Utility (AE Slow, 100% Crit chance initiate)
Her ultimate is more than strong if used correctly.
Tristana does have alot of early burst and escape/engage mechanism (Rocket Jump+Buster Shot)
I never really got to play her, but i've seen alot of people play her well and dominate with her.
MF is quite strong early on with a very strong Harass, her ultimate Dominates teamfights from lvl 6 to 14~, her MS can come in quite handy.
Twitch is (in my opinion) not a too strong laner as he has no harass, his ganking and 1on1 abilities though are quite high, his ultimate isn't as strong in teamfights as MF's early on (my opinion) but scales much better into late.