Once upon a time, I was taught to spread drive orders for at least around six disks or so to at least two vendors, three vendors if there are more than 20 disks. Use the same make and model of disk, but source them from different places. When installing the disks, make sure each RAID array has a mix of disks from each of the sources.
The purpose, of course, 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 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?