In Oblivion if you had a weapon out while talking to someone it lowered their disposition towards you. This may be why you're asking this question :) This could be used to get a higher rating with that person by persuading them with your weapon drawn, then when you put your weapon away it would bump it up higher than you could normally get it.
Since they took out this mechanic in Skyrim, and because talking with NPCs is in real time (meaning sometimes you need to be at the ready to fight), having a weapon out doesn't effect their disposition toward you, but it may change their dialogue.
I just tried selling to a merchant with my sword drawn, and sheathed. Both gave the same prices. I recommend 'sheathing' magic in town though as I've set many a villager alight on accident :)