From what Ive read here and given the time, paitence and skill, naxxcan be 10 boxed. There are a few posts about boxers getting into naxx and clearing bosses. I know there is a video of a boxer doing the 4 horsemen in naxx here somewhere. Ulduar is a little different. still to new and toons need top gear to preogress in there. eventually im sure it will be 10 boxed but its prolly gonna be a while before we see that. You can get into guild that know about boxing and hit that end game content. Depending on your team of choice, you most likley would be able to box with the guild on instance runs. I have a friend that boxed 3 spriests to 80 just for kicks. He runs naxx 10 and 25 man with his guild all the time and has always been allowed to bring his 3 spriests.

So ya, you can 10 box naxx, Ulduar one day. But if you really want to make the step to 10 boxing I say go for it.