Regardless of the fact that HTML5 coded in "loose" format will be accepted by the browser - there is no reason why you shouldn't aim for and produce valid well-formed markup.

Similar to XHTML, I would recommend that you always format your code cleanly for ANY web development whether HTML, HTML5, or XHTML.

In short, I recommend you do the following:

• all tags in lower case (no exceptions)
• all attribute names in lower case (exceptions allowed to make IE work where necs.)
• all tags closed (either self-closed or with a closing tag) e.g. <br/>, <hr/>, <img/>, <li>...</li>, <option></option>, etc.

On the JavaScript side, be sure you are using supported DOM methods (e.g. No document.all garbage) and ideally make use of a JS library like jQuery to help ease the pain of x-browsers differences in DOM support (*cough*, *cough*, make IE behave properly)

HTML5's new elements, tags, and "loose syntax" will not technically bring you down to horrible coding practices. It really depends on the coder and how their coding style generally is. You can have a coder or developer that is strict on clean markup and you can have others that can pretty much careless about clean or valid markup. With that said, coding practices is within each individual, and thus, HTML5 should not generally increase or decrease the amount of people with horrible coding practices, and it shouldn't generally alter yours if your coding practices or standards are favoring.

Regards.