Rocks are nice, but with all the plants going in there they wouldn't be very practical for planting. Rocks also add just a tad too much hardness to the final design. I have that big cement pad for the eating area and a second, separate, "sitting room" made of more block so need to balance things a bit more.
Honestly, I'm not a big fan of mulch, but unless I just pave the whole thing or just have trees poking out of the grass I don't find a lot of appealing alternatives. The key is to keep it looking good with a balance then I'm more okay with it.
Bart