Most of us are sending our CW via the computer (and logging program). I think that spaces are as important as content.
Sending "5nn (space) 4" seems smarter than sending "5994".
Good point. I've used contesting programs that tended to hose up CW spacing, too. One in particular tended to run together the K and the Ø in my callsign, so I inserted a space. K (space) ØRS. Looks funky when typing it out, but it sounded natural when played back.
Personally, I never send 5NN 04, only 5NN 4. Everyone knows what it means. Competitive contesting is about increasing efficiency without obscuring meaning. Watch out for 5NN A4 or 5NN A5 from Europeans, as "A" is the cut version of the numeral 1. Likewise look for 5NN N from some Zone 9 stations.
Also, don't repeat callsigns unnecessarily. It's redundant if the other station has copied yours correctly, and he already knows his! K5TR's example is excellent.