But that assumes that accents are absolute in some definable way.

You cannot draw boundaries round accents. They are affected by many things, some learned in the cradle, some changed deliberately, some as a result of the way one's head is constructed, or what one watches on TV, or how a partner speaks, or where one moves to.

Some people do speak 'sloppily,' but that 'sloppiness' may be common to a community, making the 'sloppy' accent a small but valid subset of what you might accept as a 'proper' accent. Furthermore, as I said earlier, like language, accents are constantly evolving and developing over time. This 'sloppiness' part of that process.

Matt is so right. If you want to avoid being thought of as 'sloppy,' and/or you want to be understood, you may need to change how you speak for certain listeners, but that doesn't mean that your natural accent is 'sloppy.' Just that someone else may not be able to understand it, or may put you into a box that you'd rather not be put in. It's up to you.