Ground beef at home and food safety
I will disagree with the other answer for one simple reason -- the fewer steps taken en masse, the lower the risk.

Now of course, this assumes that you're correctly cleaning your grinder, but because you're only grinding one chunk, or maybe a few chunks of meat, you only have to worry if those chunks of meat had contamination.

For a larger operation, every piece of meat that came before it since the grinder was last thoroughly cleaned could possibly contaminate the ground meat that you've purchased.

This of course assumes that the large cut of meat hasn't already been infected because of the butchering or some other earlier processing step.  (such as if many butchers are then feeding into a single grinder).

Now, is the change in risk enough to worry about?  Probably not, but it exists, however slightly.  If you want to be really paranoid, sear the outside of a roast, then trim them off, then grind what's left.  But of course, if your grinder isn't sterile, you're just shooting yourself in the foot.