All the places I've bought fabric recently cut them anyway, so I don't have to ask, I haven't really thought about it, but I do prefer it that way, but it's nothing to do with the grain, it's simply that when you tear you get more loose thread and these tend to tangle in the bag, on the shelf, when being washed and so on.
When I'm dealing with fabric myself, I don't have the luxury of a huge cutting table, where I can roll it out with a groove for the scissors and I tear and don't seem to have an issue with it, when preparing a backing or extracting a manageable piece. If I'm going straight to cutting, the quick handling means the tangling is not an issue and the waste is minimal, I'd waste more through not being able to cut accurately with available facilties, for a backing, after I'd basted, I will go round and cut with scissors so the tangling isn't an issue.
To me, the cutting in the shop isn't really a grain issue, you don't take there cut as something you use, it's just how they give you the fabric off the bolt, most seem to be pretty good at giving you that extra inch so if it's off, you still get your yardage, even with washing, only once have I ever had a piece of fabric come out shorter than I wanted and it was only
1/4 of an inch, but it was neatly cut and all useable.
Anne