(c) keep small notes - and if the taxi driver takes a note and asks if you have anything smaller get the note you have given back first before handing over more notes.
Hmmm... now I read this, I remember someone talking last week about a scam involving TL50 notes and TL5 notes, which are similar in colour. The story goes that the passenger gives the driver a TL50 note, and the driver does a quick switch around of notes while appearing to be looking for change, and then presents the passenger with a TL5 note (from up his sleeve or wherever) and tells the passenger that more is needed because the fare is more than TL5.
That's the story: I don't know if it happens, or if it happens often. I've never had any real problem with taxis in Istanbul apart from, as above, genuine vagueness about where something is. But it always pays to be vigilant, I guess.
And as in any country where you don't know the ropes/don't speak the language, it is a good idea to have small-denomination notes in preference to large-denomination notes if you can.