I will be in Istanbul for a week.
Do I have to tip in restaurants or not?
I'm in Istanbul right now, and when I got here I asked a friend who has been here before. According to her, no, it's not required, but is always welcome. I have not got any noticeable negative reactions over the past week from not tipping, and very gracious thanks when I have done so, but I'm still new to the country and may have trouble 'reading' people if they are being polite when I haven't tipped. Usually it's been when I haven't liked the service or felt it was already very overpriced for what was offered - not unusual in extremely touristy restaurants, for example, which you may want to avoid.
Corrections welcome from native Turks, of course :)
I just got a bill a minute ago for 18 lira for a coffee and two beers, and paid with a 20 lira note and no change. 2 lira = 1 dollar US/CA/AU - honestly, at that amount, it barely matters.
"keep the change" will work in many situations, or simply leaving the change on the table. There is no percentage expectation whatsoever. In Turkish culture, tipping is more of a nice gesture than social obligation.
As mentioned in other answers, it will always create a nice reaction from the serving staff or owner (if it's a small place), and they may even do a small gesture (offer candy, etc.)
I went to reasonably cheap restaurants and I tipped a bit, depending on how I liked the food quantity/quality. I always got nice reactions. Also, food is reasonably cheap anyway, so a tip will not make much difference.
It really depends.
If it's a high or mid-range restaurant, you are not obligated to but it is considered 'nice' to leave a tip since you can afford an expensive meal. At most cheap eateries where food costs a few lira, you aren't expected to tip. I mostly just left the coin liras as tips whenever I felt uncertain about how 'posh' a restaurant was.
In most of the restaurants or cafes, there is no expectation of a tip from the table.
In addition, in touristic areas the prices are very high. The local people and the tourists are seeing different menus, where of course, the touristic prices are way too expensive.
So, especially in those places you should not feel obligated to tip, but if you like the taste or the atmosphere or even the service, you are welcomed :)