There is no Istanbul dress code or requirement to dress conservatively. Women generally wear their everyday clothes as they would in their home countries. When visiting rural areas in Eastern and southeast Turkey, as well as when visiting mosques, it's a great idea to wear respectful clothing.
Tipping is a personalthing. I always put my tip in the tip box for alacarte and buffet restaurants. Tip the waiters bar staff early during the holiday. 20 lira is a good tip and you won't feel the need to tip so often.