I've been reusing yeast for years, both in beer and in mead. With US 05 you should have a fairly well compacted yeast, meaning you might have to swirl some liquid into it to get it to pour. What I do is to leave as little beer/mead as I think I'll need to get something to pour, swirl it around, and then save about 1 cup. If the yeast is fresh, that's way more than is needed. If it's sat around for a week or two after collecting it's still enough. Any longer than that and it will need a starter. With a yeast that doesn't form a nice yeast cake, like k1v, for example, I generally collect twice as much by volume (i.e. 2 cups), just because the amount of yeast in the slurry is much lower.
I don't know if there's anything particularly wrong with reusing the whole cake, except that I have read in several places that overpitching yeast can be as bad as underpitching. I've never had a problem with overpitching so I don't know what problems people are talking about. My using only 1 cup from the yeast cake (for 5 gallons) is because I don't really want to find out the hard way that they were right. The one thing I can say is that if you take the yeast right after fermentation has finished, 1 cup of a thick slurry will be more than enough.