It depends on the particular kit, but generally, I do. I like to have a human figure of some kind, to help give a better sense of the scale of the aircraft, and if the model is finished as an action model, like an in-flight model, or an aircraft taxiing on a runway or carrier deck, it looks better with a pilot in the cockpit.
Of course, I can just as easily place a figure next to the model, rather than in it, bearing the same points in mind.
Having said that, I did leave the excellent pilot figure out of my Eduard 1/48 P-39Q, so people could see a little more of the cockpit details. But I'll use him in some other aircraft, eventually.
I know some modelers feel that they are "ordnance" modelers, who can achieve fantastic results with a model of a machine, but that they can't finish a figure. To those of you who feel that way, I encourage you to try. You probably have the skills and tools that need to do the job, but you won't know until you try. Especially in smaller scales, like 1/48 or 1/72. You can achieve very good looks washes.
Regards,
Brad