90 would be a bit overkill I'd say.
I would agree that 30° is not enough to be safe though.
I wash at 60°, as most modern fabrics don't resist well at 90°, but between the air killing most virus/bacteria in a non hospitable environment in a few minutes, the washing process and products, and ultimately the drying process, I am feeling confident that I offer safe and clean towels.
Also, by all amounts, I keep customer's set of towels and my personal one separate