If you want to a staging area in Earth orbit, you can choose LEO or higher orbits. You can lift the greatest amounts of payload to a low Earth orbit such as the orbital height as ISS. One problem with this low orbit is the there some atmospheric drag which requires reboosting. The benefit is that it requires to least delta-v to get to this destination and the least delta-v to de-orbit from. Also at this orbital height you will have to least radiation- cosmic and solar flares. Putting ISS into a higher orbit [say 100-200 miles higher] would reduce orbital drag, but losses from orbital drag is probably not as significant as the added delta-v needed to get to the station and leave it- and of course there would be slightly more radiation. So if you were to stage in LEO, then it would probably be at around ISS's orbital height. The next choice would deal with inclination. The best inclination to from KSC is 28 degrees, launching to 51 (ISS) from KSC lowers the payload capacity of the Shuttle. But you *can't* launch to a lower inclination than the launch site's latitude, so the Russians couldn't put payload in a 28 inclination from their launch sites. So for a LEO Mars staging area, you would need to choose a inclination depending on the highest latitude of the parties' launch site involved in launching to it.
For destinations beyond Earth, you are paying a penalty in delta-v by first going to LEO, roughly, 9.5 km/sec to get to LEO and then 3.5 km/sec to go from LEO to escape (13 km/sec in total), whereas to go directly to escape requires around 12 km/sec. Plus if you are staying in LEO for any period of time then you have to also add in the losses from orbital drag.
Now to get an Earth orbit which does not have same type of delta-v penalty as LEO, you would need a highly elliptic orbit (it's vector is similar to the vector in a escape trajectory)- such as GTO. [GTO doesn't make a good staging area because as a orbit it's passing periodically the through the Van Allen Belt.] GTO is used to get to GEO (or more correctly Geostationary orbit). At/near apogee, rockets are fired to change inclination and circularize the orbit. Note: For the amount delta-v used to circularize the orbit, if instead it was applied at perigee, you could easily get an escape trajectory. From GTO it takes a small amount of delta-v (around 1/2 km/sec) to get on a trajectory which intersects or passes near the Moon. If you can pass near the Moon you can use the Moon's gravity to change your trajectory (gravity assist). Instead of using the gravity assist to leave the Earth system, you can use it to circularize your elliptical orbit. Which means you could use a gravity assist to get to a Geostationary orbit like Asiasat did. Or go to any L-point, or any very high Earth orbit or lunar orbit. Now, I "like" Earth/Moon L-1 (which doesn't mean that I think that you would *have to* always stay there).
There are a number of ways of getting to L1. One way would be to bounce using various gravity assists which would circularize your orbit so you slowly enter L1. A more direct route would similar to Apollo's trajectory- you simply wouldn't apply enough braking for an lunar orbital insertion. Another way might be get near the moon then circularize your orbit- like you would normally get to GEO. Another way might be to first go to L2, then pass back by the Moon to L-1.
Now, a big advantage of using "high earth orbit" is that there are so many options (which could also be viewed as serious disadvantage if you consider the horrible incompetence of govt decision makers "working" politics and top down command and control.) One example, from High orbit, ion proplusion would have many advantages for moving around in high orbit- and finally going to Mars or other locations. Nuclear proplusion is more doable. And of course, using lunar water and/or resources from NEOs. Also with more commerical activity in space, one problem could be the addition of space debris. Space debris in GEO isn't nor ever will be a significant problem, and in the other high orbits it's even less of a problem.