The verdict, as reported on by Anne Kadet of SmartMoney in her December, 2010 "Tough Customer" column, is this: Don't just stick on a single first-class stamp, but go for a combo of little stamps that add up to the correct postage.
According to the column, donors will assume that "some sweaty volunteer made a big effort." Instead of perceiving your mailing as just part of a mass, automated process, they presumably visualize a real person, laboriously counting out postage just for them. I wonder if the donors also worry that the charity is so underfunded that its volunteers, getting sweatier by the moment, must spend time pawing through desk drawers in search of old stamps, unable to go out and buy new ones. (Maybe desperation sells after all!)
Perhaps you could even do as my parents used to, and go to stamp stores that sell
historical U.S. stamps, unused, for very little more than their face value. Those always inspired comments in the mailroom of my college dorm.
Then again, last I heard, donors didn't mind supplying their own postage for the return envelope. So why waste the 44 cents in the first place? But maybe you could put all those stamps on the outer envelope . . . .oops, we'd better wait for someone to study that one.