Say I have all of my nexuses on hotkey 4. I double-tap 4 -- how does the game decide which nexus to center the camera on? Is there a way to quickly toggle around, centering on each building in a hotkey group in turn? Or must I always use separate hotkeys for each nexus?
I'm not sure if you are looking for this exactly, but here is my best guess. Mkaito may be right. It may very well center on the one closest to where your screen is. I will test this shortly and update my answer.
However, for quickly switching through bases you can use the backspace key or you can move that key in the settings to a more favorable position.
Spacebar is simply for recent alerts.
Edit I just tested out the theory. When double clicking your hotkey group it does snap the the building closest to your current screen position. There is no way to toggle through the list, however. Tab does not work nor does spacebar or backspace.
It centers on the one closest to where the camera currently is. Same goes for control groups with units. One thing I haven't experimented with is what happens if a fraction of the control group you're double-tapping is currently on screen. My guess is that it goes the the nearest off-screen entity in that control group, but as I said, that's a guess.