Usually its better to have 10 deg margin from the rated to the practically observed case temperature for you go for conformal coating. What you measured is the case temperature of the opto. +100 deg is the maximum operating temperature (i.e the ambient temperature), junction temperature would be higher, typically 125 deg C. Case temperature normally would be slightly less (10%-15%) than junction temperature. Since you have measure 91 deg case temperature at 70 deg ambient, you can easily go for conformal coating.