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.
travel.stackexchange.com/users/1841/David
"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.)
travel.stackexchange.com/users/3614/inchlk
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.
travel.stackexchange.com/users/3582/Michel Keijzers
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.
travel.stackexchange.com/users/108/Ankur Banerjee
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 :)
travel.stackexchange.com/users/19542/Cagri
I will be in Istanbul for a week.   Do I have to tip in restaurants or not?
Should I tip in restaurants in Turkey?
travel.stackexchange.com
<turkey><restaurants><tipping>
travel.stackexchange.com/questions/10710
travel.stackexchange.com/users/10051/Nean Der Thal
Tipping is considered a nice gesture rather than an obligation, and cheap eateries in particular will not expect you to tip. Tipping will often create a nice reaction, but you are unlikely to receive a negative reaction if you leave without tipping. Turkey is not a country with a tipping culture. In fact, tipping is seen as a nice gesture but not an obligation. Some customers feel the need to leave a tip in a more expensive restaurant. However, there is no percentage, leaving the change from the bill is fine.
People fell more obliged to tip in more expensive restaurants than in cheaper places.  There is no tipping expectations in Turkish culture. It is seen as a nice gesture. Leaving the change is fine.