Delivery guys are usually (not always) thuggish brokedicks who are on the shady side already. Keep an eye on them when they are in your place. If they keep looking around at what you have or want to use your bathroom then you might be in trouble. If they are doing this they are casing your place and will be back for a visit when your not home. If you tip them that shows them you keep cash at home and may pay you a visit if they need a quick crack fix.
Then again if you don't tip them they make come back out of spite and take their tip in the form of your computer, flat screen TV, PS3 and any thing else they can pawn or sell on craigslist for a few bucks.
I say tip them before they come in, or have them leave the sofa outside and carry it in yourself.