It doesn't need to have a double roll. I expect you have enough gear to choose from by this point, that you can afford to sell items that have "bad" substats, so you should never get "bad" rolls. I also don't think you're far enough to be able to say "well, I need a perfect set of rolls".

Say, for example, you have CDMG gloves with ATK%, CR%, and ACC substats. You probably don't *really* want the roll to land on ACC, especially if you're not using them on someone who is running against super-high resist situations.

But at the same time, if you roll it to 12, and you get a double roll on ACC, it's still not useless. You may end up finding it usable on someone else, or you may end up using it anyway, and just reducing your need for ACC on other items. You may also just get a single roll on all three substats. That's just as fine.

The only exception to this rule, for me personally, is rare speed/perception/swiftparry-set items, where I'll roll it to 8 and sell if it doesn't land double on speed substat. But that's a very specific exception. 

I keep epic gear with 1 bad stat, lego gear with 2.  If it hits a bad stat then I sell it.  Should I sell that type gear before I roll it out to 8?

Rolling to 8 is only a small investment in silver, you can allways try your luck before you sell. I would definetly keep epics with one bad and two good stats and roll them to 8 before selling. If you have an epic with, for example, substats spd, critrate and flat def and at lvl 8 you hit speed and/or crit rate, it's definetly ok using that item.

Base it on what you have to choose from. If you find you don't have enough items to upgrade because you are being too restrictive in your selection criteria, then loosen it. If you find you have too many items to upgrade, then tighten it.

I need to tighten.  I have too many.  I am stuck on selling high grade gear from early days in the game.  

Here's a few videos with some good gear cleanses. Perhaps they can help give you some case by case examples of what is worth keeping versus not in your own account :)

CLEANSE YOUR GEAR! (What do I upgrade?) https://youtu.be/-SxEu5Fdohc 

Accessories are tough to know if you keep or sell. WATCH THIS! https://youtu.be/eNdGS6HIn_w 

Another gear cleanse, since folks are always asking what to keep or sell. It's on a takeover, so there's lots to go through there. https://youtu.be/0lbAMJvHLUw