Tipping in Barcelona is the same as the rest of Spain; not compulsory but certainly appreciated. It is also a well-known fact that Catalans tend to be more rigid in terms of their spending, therefore the Barcelona culture does not advocate major tipping tendencies. To add to that fact, people employed in the service industry in Spain (and most of Europe) do earn a wage, which means they do not rely on tips as their means of survival. This can be reflected in the quality of service as well!
However, the general rule of thumb is to tip those in the service-related industries, like waiters, taxi drivers (on longer rides, such as to and from the airport) and gas station attendants. The standard for tipping is usually between 5-10 percent, and the 10 percent is usually reserved for when you eat at a high-end restaurant. Taxi drivers and waiters can always spot the foreigner (and happily so) when they leave them large tips.
The local Goods and Services Tax (IVA) should be included in the bill you receive, so there is no need to calculate any extra costs.
Other services like hairdressing, manicures/pedicures and massages do not require tips.
If you have lived in a neighborhood for some time, perhaps one that is serviced by bottled gas as opposed to piped mains, or if you have a doorman or service person at home, it is customary to give them a small gift at Christmas. This can be in the form of money, or a small gift of food or drink. However, none of this is compulsory but a well-practiced, courteous gesture.