Tipping is strictly optional in Thailand.
At many hotels and hotel restaurants or other up marked places your will find at the menu / price list notes as 456 THB ++. Restaurants and hotels often publish only the net prices and when you pay your bill you will find a 7 % VAT and a 10% service charge on the bills. In this case a tip is not necessary.
Give a 10% tip at restaurants if a service charge is not included. Tip porters and bag boys between B20 - B40 tip. Do not tip taxis, but you hmust pay for the expressway tolls.
7 % VAT is on all goods and services which includes meals and rooms.
Selected department stores offer refunds for tourists, but VAT is generally non-refundable.
Guides or crew from tours are expecting a tip, since their salaries are mostly not so high. But also here use your opinion. If you wish to show you appreciation for a good service go for it!