Maintenance for the week of July 6:
• PC/Mac: No maintenance – July 6
• ESO Store and Account System for maintenance – July 8, 10:00AM EDT (14:00 UTC) - 12:00PM EDT (16:00 UTC)
It hasn't happened for a while, but Steam/ESO have had login issues in the past that prevented Steam-only PC players from playing while non-Steam PC accounts could log-in normally.
So unless you have a compelling reason for adding ESO to Steam, its probably not worth the risk.