Case makes no difference
Somebody pastes an address with capitals in it and worries they have the wrong one. They do not. Case is discarded before anything is looked up.
Bucket: ALWAYS
The client lowercases the whole label before it does anything with it. An address written in capitals, in mixed case, or in the odd case that a phone keyboard produced, all resolve identically.
Why it works that way
Two reasons stack. Hostnames in general have always been case insensitive, and base32 as defined uses one case with the other folded in. So there is no layer where the difference could survive even if somebody wanted it to.
It follows that the number of distinct addresses is not doubled by case. Upper case A and lower case a are one symbol, not two, and that is why the alphabet is 32 rather than 58.
What this saves you from
Autocorrect. Phone keyboards capitalise the first letter of anything that looks like the start of a sentence, and chat clients sometimes capitalise after a full stop. If case mattered, half the addresses passed around would break on arrival for a reason nobody could see.
It also means you can compare two copies of an awazon market address without worrying about how each was formatted. Lowercase both and compare. Any tool you use to diff them will agree.
The trap hiding inside this
Case insensitivity is exactly what makes lookalike characters dangerous, and people rarely connect the two. Because the client folds case, an address written in capitals shows you I and l and 1 in fonts where they are hard to tell apart, and the folding means the capital form is a legitimate way to write a real address.
So an address presented in capitals is not suspicious in itself, and it is harder to read. The right response is to lowercase it and compare, not to reject it and not to trust it.
A mistake worth naming
Some people believe that a mixed case address is a vanity feature, like the mixed case used in some cryptocurrency address formats where capitals carry a checksum. Onion addresses do not do that. The capitals carry nothing. There is no hidden validation in the pattern of upper and lower case.
The checksum in an onion address is two bytes buried inside the encoded data, not something visible in the letters. The anatomy page shows where it sits.
A small thing this buys you
Reading an address aloud gets easier. There is no need to say capital or small, because the distinction does not exist at the far end. That matters when two people are checking a string between them, which is one of the few situations where reading character by character is actually a good idea.
The convention people use is to spell the letters and say the digits plainly. Since the alphabet excludes the four confusable digits, saying seven or two is unambiguous. The only pairs that still cause trouble are the letters that sound alike, which is why the alphabet with words for each letter is worth knowing if you do this often.
What would move this
A change to the hostname layer that made labels case sensitive, which would break decades of software and is not on anybody's roadmap. Or an address format that used a case sensitive encoding, which would be the same protocol change that would move the alphabet statement.
Until one of those happens, treat case as noise. It is the one part of an awazon onion link that genuinely does not matter.