Quotes are always required. Now, the following will give an error (Error: an attribute value must be a literal unless it contains only name characters):
<table width=100%>
Now, the following does not give an error with validator, not sure exactly why as these HTML specs are so darn complicated as there are a million different ones, but it has to do with being XML compliant :
<table width=600>
Quoted from the specifications on HTML4 :
"4.4 Attribute values must always be quoted"
See :
http://www.w3.org/TR/html/#diffs
So, it's best to always use quotes.