Shouldn't you look in both guides if you are sure of the multiclass? When I do a multiclass I normally think first on what is that character focus. In your case, is he a caster first or your focus is getting a mean Sneak attack in? In your example, as far as I understand, your focus is the latter, so I'd advise you to look in the Rogue guide to...
After all that was said here I don't understand how one could still argue that the grades to non optional class abilities isn't usefull. There were multiple examples of how people actually used them in usefull ways.
Sure you can still not like them, it's your right to do so, and you can also not find it usefull for what you want to do, but there is still a diference from, "I don't like it", or "It's not usefull to me" to "There is no use for them". If even one person found use to it (there were more than one examples of uses here) there is already proof that the grading is usefull.