Should you salute an American Flag when you pass it in uniform?

I saw what had been sent in for an answer, and understand, the answer obviously makes sense, of course, certainly, however, in any event, I did find it an interesting question, regardless, certainly...in a biopic on Lincoln with Henry Fonda, called, I think Young Mr. Lincoln, there was an interesting depiction of Lincoln riding into Springfield, IL, passing the courthouse or town hall, tipping his top hat, done by Henry Fonda, I think it's on YouTube, your question made me tink of that scene, the bldg had a flag hanging outdoors above the door, which was stationary, I'd always found that an interesting depiction, may e it was a custom of the period, I never knew for sure....

The other answers are correct. A few nit-picky points:
(1) The Army distinguishes the "colors" from the "flag." The colors are typically only brought out for parades and ceremonies.
(2) Personnel in uniform are supposed to salute the colors at six paces and hold their salute until the colors are six paces past. The flag is only saluted when being raised and lowered (typically at reveille and retreat).
(3) RE: the Lincoln movie. If the (we'll say "banner" for the purposes of clarity) was the Colors (smaller, w/gold trim) then as they were displayed outdoors, rendering respect is appropriate. But we typically only salute flags while being raised or lowered.

When being raised/lowered or carried as part of a procession, certainly. If someone felt the need to stop and salute when passing a fixed flag (pole, building, etc) I would see nothing wrong with it.