Rashi comments that Avraham was afraid that they were ערביים who worshiped the dust on their feet and Avraham was מקפיד not to bring avoda zara into his house. Lot on the other hand was not מקפיד and therefore did not have them wash their feet first.
It is clear from Rashi that he is praising Avraham Avinu for his behavior. However, if you look at the Gemara ב"מ פו you get a different picture. The Gemara says that because Avraham was חושד that they were עובדי עבודה זרה Yishmael came from him whose descendants worshiped exactly as Avraham was חושד the malachim. The Gemara is saying that Avraham was חושד בכשרים and punished for his behavior while Rashi seems to be praising Avraham for not wanting to let עבודה זרה in his house.