I don't think you can go wrong if you open it as early as possible. My PB (a genuinely nice guy who knows his stuff pretty well, I think) told me last week that "if the dogwoods are blooming, so is your pool." He usually keeps his open all winter.
You can open as soon as you know there won't be another real freeze. You can put off opening until the water gets to around 60 degrees. If the water gets warmer than that you are fairly sure to get algae. Of course you might get algae anyway, but opening before the water gets to 60 degrees gives you fairly good odds.