Yeah it does kinda matter, which is problematic for onboarding new users. Personally:

• I read the rules/TOS.
• I checked the Instances page to make sure instances I wanted to access were linked and not blocked.
• I looked for any introduction posts to see what the backend infrastructure was like.
• I checked the username I wanted was available.

It should not be that complicated, but it also has to be unfortunately. I had to make sure all those things matched what I personally wanted. This much choice is great for heavily-invested tech nerds with free time but sucks for anyone else wanting to join in.