A human landing on Mars may require fuel to be stored in low Mars orbit (LMO). The motivation for this step arises when mission planners seek to leave the crew's interplanetary propulsion component—that is, the engines that push the crew to Mars—in a highly elliptical "parking" orbit around Mars. For a spacecraft that will return to Earth, a parking orbit is preferable to a lower circular orbit because it requires less fuel to brake into and depart from. In a sense, the vehicle doesn't have to descend as deep into the Martian gravity well. Of course, the crew will then need to travel from Mars parking orbit to LMO and vice versa upon conclusion of the surface mission.
It is in this step—getting from LMO to Mars parking orbit—that a fuel cache in LMO will help out. The amount of fuel required to make this transition can be calculated quite precisely using the Tsiolkovsky rocket equation. It will depend on the type of fuel used, the total change in velocity required, and the mass of the crew, payload, and spacecraft. Since the velocity change needed to achieve an elliptical orbit must be imparted rapidly, the only suitable fuels seem to be traditional chemical rocket fuels. The total change in velocity in an orbital transfer depends only on the sizes of the orbits. A bit less certain is the mass of the crew's spacecraft, so it's best to leave this as a variable.
It is clear from the rocket equation* that using a fuel with 300 seconds of specific impulse (Isp) to impart a 1.2 kilometers per second change in velocity will require an amount of fuel greater than 50% of a spacecraft's non-fuel mass. If the fuel used offers only 250 seconds of Isp, then its weight must correspond to a full 63% of the rest of the spacecraft.
The alternative to storing this fuel in LMO is for the crew to carry it with them to the Martian surface. This would introduce some complications to the fuel's storage: higher temperatures, larger thermal fluctuations, and the shock of landing and ascent. More important, though, is the sheer mass involved. Any mass launched from Mars's surface requires several times as much fuel just to get it into orbit. But this mass and fuel must first be landed on the surface, and this landing requires several times as much fuel as the spacecraft mass and ascent fuel put together! I think that this post-ascent fuel—which is necessary simply to get from LMO to Mars parking orbit—is too much to take to the surface and should be left in orbit.
*see Appendix A