Form Validation and xss_clean

[eluser]Madmartigan1[/eluser]
[quote author="Eric Barnes" date="1293653183"]
* Validate the data to ensure it conforms to the correct type, length, size, etc. (sometimes this step can replace step one)
[/quote]

I think this is exactly what we're talking about. Under what circumstances can step 2 replace step 1?

For instance, if one of the rules is "integer", adding the prep rules "xss_clean" would pointless right? How can an integer be unsafe? Etc...