Yes it is. Our installers used low expansion foam and did it as a matter of course.

I wouldn't use any expansion foam, but if you are going to use it definetly you want to use low expansion, DAP make a decent product in a blue can, DO NOT use the red can. Also make sure you read the manufactures installation info many with recomend not using it, some will even void their warranty because of its use.

ABSOLUTELY! A low expansion foam seals the gap between the window frame and the rough opening in a way that no other insulation can. If one uses fiberglass insulation, it will insulate the gap but will not stop potential air and water infiltration; it will just pass right through the fiberglass. A foam will seal every gap and crevice and will not allow the air or water to penetrate into the building through the gap. Curiously, there are a handful of manufacturers that recommend fiberglass insulation still; they're afraid I think that the foam will bow in the jambs if too much is put in or if an expanding foam is used by mistake.

If your installer does not use low expansion foam on a regular basis I would choose another installer. I have found it is usually a cost issue when a installer doesnt use the foam.

Not using foam on a retrofit installation is a crime. On block frame installations if the measurements are tight foam might not be necessary. I know some manufacturers claim they will void your warranty if used but this is because some installers have no clue how to use foam.