Do you put an XML declaration at the top of an XHTML file?
XML specifications require an XML declaration, but I don't usually see it on XHTML files, and my tests show not all browsers know what to do with it.
What's the standard practice regarding this?

Do you put an XML declaration at the top of an XHTML file?

No, I don't. I know, the purists scoff at serving XHTML as text, but that's on on-going argument.

The XML declaration will throw IE 6 and below into Quirks mode, so you don't want it in there at all. If you're using PHP, you can do some browser sniffing and only serve the page with the XML declaration to Standards-compliant browsers.