iMac color calibrations
I also use an iMac (a 27" model) with glossy screen. Lacking a dedicated room, ideally equipped for my image processing, I'm forced to locate the workspace within my large living room.
I've moved my desk into almost every possible position and corner of the living room without truly finding that ideal spot. Window reflections on the screen are a pain and keeping the curtains drawn during daylight hours isn't to my liking. I finally found an area, where the desk is facing into the room with a wall behind me (and no - or only minimal - reflections).
I apply color management by calibrating my monitor and using (OEM or self-created) icc profiles for printing. I soft proof an image in CS5 before printing to check for "perceptive" or "relative" color rendition. Printing from CS5 or from LR3, almost all prints were too dark! I could work around this by lightening the image in LR3 but it was still frustrating.
Then I noticed, that actually, I had processed all images (in LR3) to be too dark. What was I thinking, how did this happen?!! The desk where my iMac was placed, was facing the living room. From my desk, I view the sofa on the right and a long row of (IKEA) book cases on the left. This row of shelves has lights mounted above each shelf. I like having these lights turned on in the dark. But when adjusting image brightness on the iMac, these light sources were directly behind (in line) the iMac. It was a bit like oncoming traffic an night, I was being (slightly) "blinded".
I then rotated the iMac on my desk so these (somewhat bright) light sources were off to my left and not directly located behind the monitor. Suddenly I noticed, how dark my images were on the monitor! I also reduced the iMac brightness (actually luminance) to a middle value. Correction of image brightness suddenly gave the printed results I'd been missing. Now, prints matches the images on the monitor - at least as closely as possible.
To make a long story short, the location of your iMac, along with the environment's lighting conditions will greatly affect your image processing AND, if that glossy screen is a bother or not. Now, I don't mind the glossy screen one bit. (Actually, the screen is simple to remove. I don't understand why Apple doesn't offer exchangeable, matte, screens.)
You're on the right path to calibrate your monitor. Color management is imperative for color consistency and necessary when sharing files.