As long as you're not buying all the disks at once sticking with one manufacturer and brand should be fine. If you're buying 25% of your total inventory every year it'll all be spread out to just a few percent per month.
But when you're buying 100% of your disk inventory at once there's a serious "all eggs in one basket" risk.
But when you're buying 100% of your disk inventory at once there's a serious "all eggs in one basket" risk.