I presume that you are in the UK? If not local wiring codes in your country will apply.
The wiring regulations in the UK are now BS 7671, 1992 (the old IEE Wiring Regulations).
The Regs are non specific on the point and say simply that surface mounted socket outlets on a wall should be at a height above the floor or adjacent work surface sufficient to minimise damage to the plug or cables. (Reg No:553-01- 06).
If I were you, I would install residual current earth trips, (RCD devices) and ensure that all workshop sockets are metal clad.
Mushroom panic buttons at strategic places are also a good idea in case you have to disconnect the juice pronto.
All of the outlets will be protected by an RCD which has a separate earth to the supply (fed from the non-RCD side of the consumer unit in the house). I've had a great deal of advice from an electrician on all this - just the height of the sockets left to determine
Possibly a little OTT, but in my workshop I mounted four-gang sockets upside down underneath (at the edges of) the workbench - I was concerned about dust, shavings etc. getting into the sockets. They are about waist height. The only problem is that it's sometimes difficult to plug things in ('cos you can't see the socket) - but most of the time I have all I need plugged in already.
I put my sockets at about eye-level. This means they are up out of the way of the bench/chisels/projects etc. You don't have to move them if you replace you workbench with one just a few inches higher, and they don't take up any "space" on the bench by sticking out - and stopping projects being pushed to the back of the bench.
I'll second (or is it third, now?) the socket on the ceiling. I put one of these in when I re-wired my garage workshop and I can tell you it is really handy when you want the flex out of the way. Much less chance of a tangle.
The answer to the original question is stick the sockets wherever they will be right for you. Mine are not all at the same height. They tend to be higher over benches and lower in other places - or very high as in the case of the ceiling fitting.
Thanks everyone - I'll add a couple of sockets (or at least cable to the ceiling) up there.
I wasn't worried about the cables to the table saw and P/T running across the floor as the workshop is only 4m x 4m - I thought that having a cable hanging from the ceiling would get more in the way - is this not the experience of others that have done this?