I have used several Sharp convection/microwave ovens over the years in both RVs and Stick and Brick. I have used both wire racks for microwave. convection, and combo cooking for all of those years.
Note that the "High Mix" for roasting and "Low Mix" for baking options combine both microwave and convection cooking and a rack is specified for roasting and baking.
Of course, the purpose of the wire rack is to get heat under the item being cooked. Use the rack when that need is indicated.
since it causes no harm, we leave the low rack in the oven for storage all the time.
Our large Sharp at home has a metal turntable.
Good Luck!
Wil