Spinny,
The important thing is does the site look good in all browsers. After that you can worry about validating all your code. Here are some of my rambling thoughts:
If the doctype is not exactly as provided at
http://www.w3.org/QA/2002/04/valid-dtd-list.html then it will not validate. If you use HTML and close some tags (meta, hr, br) you may get errors in the validation that are not really errors.
Leaving the doctype off or not using the exact correct doctype will put browsers in quirks mode. Sometimes it is easier to get the page to display the way you want it to look if you force quirks mode.
Another way to not be able to validate is to use a specific program, knowing that it will not validate, but you still want that feature on your site. In that case, validating is just a way to locate errors in your code. When I am making changes to my site, I check the validation. I frequently find that I have failed to close a tag, or while editing I have cut and pasted everything I wanted to move as well as picking up a portion of a tag. As you can imagine, having an image tag that is only <im does not work so well, and the validator will catch things like that.
The CoffeeCup folks (here in the forums) are not overly concerned with validation, but some other forums will not provide any assistance until you validate your code.
Here I end my ramblings!