I'll be running some new circuits inside my unfinished attached garage in order to provide some new outlets on the other side of the garage. EMT conduit will run about 20' along one wall, then turn 90 degrees and continue another 20' along the adjoining wall. At some point in the future I may wish to add an outlet halfway along one or the other conduit run, so I plan to include inline 4x4 junction boxes halfway along each segment. The wires inside the EMT will be either 12ga or 10ga THHN/THWN. In order to make things easier to connect in the future, should I leave a perhaps 1 foot loop in each wire where it passes through a junction box? It won't be connected/cut/spliced there (yet or perhaps ever).
Yeah, that's fine. Your maximum number of circuits per pipe is four, unless any circuits are over 28 amps, then you have to do some math.
Leaving a loop in each of your "possible future tap" junction boxes is a good idea. If you splice into a wire, you need 6" of wire length in the box, so if you leave yourself 12" + a bit, you'll be fine.
Don't feel obliged to throw wires into the conduit now. If you are building conduit legally, you must pull the wires in only after the conduit is complete, so you can always add wires later. Do not build conduit that can't be pulled.