In XML, quotes are always required (since XML has SHORTTAG NO; see Comparison of SGML and XML). You may use XML in the future, and in that case your life will be simpler if you have adopted the habit of quoting attribute values.
When someone (you or someone else) later edits your HTML file, he may easily forget to add the quotes if he edits an attribute value in manner which makes the quotes mandatory. For example, an attribute like SRC=foo.gif is legal, but if someone changes the attribute (e.g. due to moving a file to another directory) to SRC=images/foo.gif it becomes illegal.