I think this is a very good question. Some recips just don't work so you have to pull out ol' notepad to fix them.
The main problems I come across are marquees with no width, so you can't read them. Too many <br>'s and code put together without even a thought given(it seems) to proper nesting.
And yes, xhtml will be the new standard, so <br> should be <br />. Same with <img> and others. I just fix them once, add them to my saved recip table and don't worry about it till I change niche. But it is the part I dread,'is this going to work or will I have to spend more time fixing it so I don't get declined for having an improper recip'.
And hey, if anyone wants to trade top listings for properly coded recips, I'm game
----chris