Yes, that will work, although I would probably use Non-Vocal. But, for me, that does not seem like the best solution. The majority of my tracks are non-vocal and it also means I have to do worry about this every time I import a new album.
Lyrics lookup and lyrics display are two totally different activities and should be treated as such.
I am sure some people like this lookup feature. But others do not.
I still think a user selectable option is appropriate, just like Auto Import is an option. The user should have the option for these types of automatic activities.
And, as long as azlyrics.com is used for lookups, I do not want to chance unknowingly hitting their site. And, people with a lot of non-vocal music will be unknowingly hitting it on a regular basis if they are not careful.
I really think a user option to turn the feature on or off is appropriate.