Once upon a time, I was taught that whenever I needed around six or more hard drives I should spread the order among at least two retailers, and three when the order gets up around 20 drives or more. Use the same make and model of disk, but source them from different places (this was back when raid cards would only work right if all the disks matched exactly). When installing the disks, make sure each RAID array has a mix of disks from each of the sources.
The purpose here is to prevent problems if the there's a bad lot of disks and the vendor just adds them to your shipment sequentially. This could result in multiple disks in the same RAID arrays failing at near the same time, which could mean data loss.
Is this still good guidance, or are most reasonable enterprise vendors (ie: not newegg, tiger direct, or amazon, that deal mainly with consumers) aware of the situation and now deal with it correctly? Is it dealt with at the manufacturer level (manufacturing runs are broken up before vendor delivery)? Is it something you have to ask for? Or am I just worrying too much?