I am hopeless at classifying what I write by form.

I classify by style and content.

I could care less what it ends up looking like anymore, as long as it says what I want to say and when read, sounds like I want

I try but I suspect sometimes I get it pretty wrong. Things like sociopolitical etc categories are a little confusing and still aren't defined, so I probably don't get those right.

I often don't know exactly where to put them, so I just do my best and leave a note in the description asking for suggestions.

I have to agree with what =DrippingWords said. Sometimes it's a tough call to decide which category something fits in best, and some categories people may stay away from, even though they might like it not knowing that it's not your average one category. But to be honest, I'm terrible with categories and labels in general just because when I go looking for music, art, books, I don't think "gee I want to read a good horror, or fantasy, or etc." I think, "I want to read something" and I never go about browsing by paying attention to what genre things fit into. I tend to like a bit of everything of almost everything. = /

I never know what to do if it fits more than one category.

I have been slowly moving stock deviations into more appropriate categories. As sub-cats are added, I do try to get things moved, but there are many I still need to move to "premades". Otherwise, I really do try and choose the right category, but there have been a couple times when I was using the "shortcut" of the popular choices that pop up when submitting that I accidentally clicked the wrong one!

I do take care to pick a category, but I think it would serve me better if I didn't.

For example, if I'm particularly looking for haiku or villanelles or sonnets (let's say sonnets), I have to browse:
Poetry > Children's > Traditional Fixed Forms
Poetry > Emotional > Traditional Fixed Forms
Poetry > Family Life > Traditional Fixed Forms
Poetry > General > Traditional Fixed Forms
Poetry > Horror > Traditional Fixed Forms
Poetry > Human Nature > Traditional Fixed Forms
Poetry > Humor > Traditional Fixed Forms
Poetry > Narrative > Traditional Fixed Forms
Poetry > Nature > Traditional Fixed Forms
Poetry > Romantic > Erotic > Traditional Fixed Forms
Poetry > Romantic > General > Traditional Fixed Forms
Poetry > Romantic > Post-teen > Traditional Fixed Forms
Poetry > Romantic > Teen > Traditional Fixed Forms
Poetry > Satire > Traditional Fixed Forms
Poetry > Sociopolitical > Traditional Fixed Forms
Poetry > Spiritual > Traditional Fixed Forms
Poetry > Transgressive > Traditional Fixed Forms

Realistically, I'm not going to browse all of those - I'll pick a few and then browse just those. And most other people looking for a particular form will do the same.

So if my hypothetical sonnet was in Poetry > Children's > Traditional Fixed Forms, then someone looking for sonnets is probably not going to find it unless they were specifically looking for children's poetry. It would be better off in Poetry > General > Traditional Fixed Forms because more people will look there.

So, as I said, I do try to categorise everything properly, but part of me thinks that I'd be better off not doing so.

I do. but I feel as if the da categories are bit limited sometimes and some of my pieces end up in general poetry because there is no other fitting category.