Maintenance for the week of May 25:
• PC/Mac: No maintenance – May 25
• ESO Store and Account System for maintenance – May 27, 6:00AM EDT (10:00 UTC) - 4:00PM EDT (20:00 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.