It's smart to launch supplies in low-energy orbits so as to increase the mass you can get to Mars for the same fuel budget. Humans have to transit quickly between the planets, but supplies can handle a much longer trip on a much less comfortable spacecraft and, to be there for the humans, they'll have to launch much earlier.

And yes, no sane human being would consider launching to Mars before a return vehicle is already settled at the destination and ready to take off. The return vehicle and its backup. And the habitats, supplies and everything else.

Make sure all things required for survival and a possible return already be there?