I use sealant, even though Shimano says not to. I use for two reasons - (1) the sealant may close a small hole while riding and prevent a flat and (2) while tubeless rims and tires should not require sealant, I find that my tubeless set up very slowly leaks air from the sidewalls such that pressure drops like 10 psi a day (similar to a latex tube). The sealant helps really seal up the sidewall so the tire holds pressure over a longer period of time. Maybe not an issue for all but annoys me nevertheless.
I would strongly suggest Stan's. I bought the Hutchinson nonsense that only their sealant should be used in their tires. However, after I spent $30 on 2 cans of sealant with CO2 and the cans malfunctioned and I got sealant all over the place, I will never use that expensive product again.