If you're talking about the UK, it varies. Pubs are almost always on counter service, bars may be, or may also have table service (or you may be presented with a bill on a tray).

'One for yourself' means, these days, 'have a tip', which can vary from 50p (the standard amount) up to the price of a standard pint (which can be well over �3). While in smaller pubs, and towards the end of the evening, staff may take the 'one for yourself' at their word, the chains take it rather differently. At Wetherspoons, for example, where every spill and spoilt pint at service has to be accounted for, all the 'ones for yourself' are amalgamated into a single drink after the shift end, if the server wants, and all the rest offset against the spoilt drinks.

Disgraceful, and a very dubious process. Any offer of a drink is surely made to the employee and not to Weathersppons to allocate on behalf of the employee after spilt pints etc are taken off.

Would you tip a barman personally if you thought that the company takes the gift and uses to offset their losses

Exactly ^