Risky, depending on when you do it. There are thousands of guests actively using the system at any given moment. If you drop a rare Fastpass, and someone else happens to be searching that same time, it will be offered to them.
While it is being offered to them, it is taken out of general circulation and nobody else can see that Fastpass until they navigate away. (This is to reduce the chance that multiple people are offered the same FP and causing disappointment.) Once they navigate away, then the FPs they didn't pick are thrown back into availability for the next browsing guest.
This is the craziest on the actual day of usage. It's is why you may be offered a different set of selections every time you refresh your browser, including rare Fastpasses one moment, and none of the rare ones the next.
If you're doing it 50 days out or so, the risk is lower that someone else is browsing that exact day, park, and hour, but there's no guarantee you'll get it back.