Tipping is a relatively new concept in Armenia, with little tipping during the Soviet era. It has become pretty well established in Yerevan, especially in restaurants and cafes, where tips are now usually expected. Anything from 10-20% is ok. On smaller bills, just leaving the change may suffice. Taxis may or may not expect a tip. Drivers will sometimes say they don't have change, or enough change, in hopes of getting to keep the difference. Outside of Yerevan is a different world. Those who have experience with Westerners will probably expect a tip, while those who don't probably won't.
Some workers, like at cafes, are said to only get paid in tips, but you should never tip unless you want to.
NOTE: Some restaurants add an 8 to 10% "Service" charge to the bill. This is *not* for a tip. This is a separate charge that the restaurant keeps, for no apparent reason. Ask about this type of charge, or look carefully on the menu, if you want to avoid places which have this. If you want to tip, you must leave money separate from this charge.