I don't see 10/10 as 'perfect in every conceivable way', mainly because marking stuff out of 10 will inevitably result in a bit of rounding when judging stuff. I follow this film critic on Twitter that gives marks out of 100 and, out of the 6,000+ films he's reviewed, he's only given 3 a perfect score. To me, that implies absolute perfection and absolutely no desire to change a single thing. Whereas 10/10 just doesn't have that ring of absolute perfection to it: it just means very good in comparison to everything else that is out there.
Which is why I'd give Cowboy Bebop a 10/10, but probably not 100/100. Judging it as a whole and not a collection of episodes, it's a remarkably cohesive piece of art. Sure, there are a few dud moments and some episodes work better than others (hence why it's not perfect), but everything comes together remarkably well, the animation is really good, the characters are memorable, it doesn't outstay its welcome, the music OMG THE MUSIC. To me, it's the best thing that the medium has done, so it deserves a 10 very easily.
Avatar: Miss Bellum from The Powerpuff Girls | Courtesy of: Harlequin