Short answer, no I don't use a grate.
Long answer.... when I got my Roti, the wrong size was sent (the debate is still out whether it was ordered wrong or sent wrong, but that's another story) so instead of sending it back, I used it as an excuse to get another kettle!!
I found an 18" on CL for $25! It was in good shape on the outside, but the charcoal and grill grates were rusted out and beyond repair. SO, the 18" became the dedicated Roti and I only use the charcoal grate. (didn't have to buy one)