I think they both have their uses.
In my experience 'Optimistic' effort estimates can help you get through the door with a new client. Even though overruns in such circumstances don't make anyone look good and often threaten extensions, in an emergent relationship there are far more unknowns than in an established one, so you have a more extensive list of excuses. Once you get over the initial overrun scenario you can begin getting increasingly more 'realistic.'
'Realistic' effort estimates in turn can be better for established relationships where both parties are over the honeymoon period, there is value locked into the relationship and obtaining grudging permission is slightly more feasible.