you use what ever you need to farm.
Ideally, AAs are enough, but if one drops low out of your reach, don't hessitate tp Q.
If two drop low at the same time, Q is once more your friend, or the AA reset of E.
If your under tower and would need two AAs after the tower shot, useing AA+Q or AA+E isn't a bad idea.
Generally, your order fro CSing is AA>Q>E>W, but overall don't hessitate to use your skills(well, asides from W) to farm unless you can dominate the matchup and thus want to use your skills for harras/trades.
But since all the nerfs, you want to farm up till atleast lvl 6 with fiora in most matchups, often even till your first big item is complete.
Generally speaking, there are no really impossible matchups, thoguh there are many rather skilldependent ones like irelia or riven, and many where you have to play very carefully early on but outscale later like poppy.
The two worst matchups from my experience are malph and lissandra(if played well). You can farm savely vs malph, but his midgame impact is just much higher than yours and you will hardly have a chance to snowball vs him. And later when you want splitpush, malph will be able to initiate 5vs4 fights vs the rest of your team with his ult rather easily.
Lissandra is harsh in lane, and if well played can always hold her own vs you even later on vs the splitpush, while her tp flanks are far more devastateing than yours just like her midgame teamfighting.
And if she should for some reason loose the 1v1 vs you, she's nearly as good as malph at forceing teamfights.