I recently constructed a built-in desk and bookshelf in my upstairs
loft.

There was an outlet on the wall that happened to be right where two
bookshelves joined together. I had to remove the cover plate and
outlet in order for the bookshelves to be flush against the wall. I
removed the socket and rejoined the wires together using wire nuts. I
then pushed the wires back into the box and put my shelves in front of
it...without using a cover plate. Its this last part that I'm kinda
regretting now that the built-in is complete.

Should I have put a cover plate over that or will the wire nuts be
sufficient? None of the wires were anywhere near crossing each other
but I still have a little anxiety about it. I'm pretty sure I know
what the pro's would do...

Opinions? I'm probably breaking major building codes doing that too.