If they were stored where direct sunshine got to them often, or high temperature often, or very dry conditions, I would not use them. The best way to store them is to reduce the pressure to about 4 to 6 PSI and put them on their sides.
I also put several red bricks under the tire on the bottom of the stack to help air circulation, but that is mostly to help prevent the rims from rusting from too much humidity.
I understand your concern about the tires being 6 or more years old, but winter tires do not get exposed to the high temperatures and the high sunshine like summer tires.
The main concern would be damaging them during mounting them because of their age.
I got a second extra set for one of our cars when my brother took an old Buick to the bone yard. The rims were shot because he stored the winter tires on bare earth in his back yard with a thick tarp on top of them. The new rims that I put an extra coat of Rustolem flat black on and put the tires on a year before we gave him that old car were totally rusted out. But the tires are still good. I had them removed from those rusted rims and then sprayed Armor-All on them and stored them without rims for three years before using them. When I had them put on another set of rims I had for one of our cars, the place that mounted them said there was a chance that they would be damaged when stretched during mounting, but they survived that unharmed.