Locals often tip even if modestly, it is considered polite and the Spanish are sticklers for manners and etiquette. However it isn't like the USA, and there is not an expectation for tips. Leaving the small change behind to round up to the nearest Euro is the most common. Maybe an extra Euro or 2 if the service was very good and added value. In upscale restaurants then tipping can be expected.

Outside the restaurant business, some service providers, such as taxicab drivers, hairdressers and hotel personnel may expect a tipping in an upscale setting.

There are no added service charges on the final bill at any hotel restaurant or bar.

If dining on the menu of the day, no tip is expected or given by locals.

if you are uncomfortable with tipping then just do what the locals do and leave the change discreetly under a plate or something. Don’t forget these service staff work hard for little money and any extra is always appreciated.