When you have E-8s who are not in compliance with the regulations, is it OK to correct them on their appearance?

Yes, you can, but be Careful and Diplomatic. MAKE SURE YOU ARE 100% RIGHT FIRST......Most Master Sergeants and First Sergeants dont respond well to smartasses...

My intent with this post was to get good solid advice not because I wanna be command major private , I just feel like if your a senior leader why should you stop following the regs ? I mean it probably shouldn't come from a private but I needed advice and I got it. Iam aware I have spelling issues but that's a whole different topic . If this was a essay I'd be alot more cautious. Iam a private looking on how to grow to be a good leader not be taught that when you are higher ranking that you can do whatever.

Like Roger said, be 100% right, and have the regulation on hand, on paper. The regs are not wrong and having it on paper shows you've done your homework. Additionally, ask if you could speak to the MSG privately. "Praise in public, punish in private" is something that's taught to NCO's, but it provides guidance for situations like these. While you can't punish in any way, asking the question in private is the way to go.

I have always found if I have seen something that needed correcting with a senior enlisted phrase it in the form of a question. They are generally more likely to respond and correct the issue. They will also be less likely to be chewing your butt for doing so as well. Because it puts in the form of you seeking knowledge.

I don't see anything wrong with doing a quick on the spot correction for things like boot laces, pants unbloused, or a jacked up name tape. I have found those types of corrections are welcomed as long as it is done tactfully. Don't walk up to them and rip their name tape off and tell them they are jacked up. You will probably end up in the hospital. Proceed with caution.

If you intend to correct their spelling or grammar, I'd suggest you NOT do it in writing. At least not at this point in your life.

Ask to speak to them one on one off to the side and be brief and tactful about your observations. If you try and do it when they are in the middle of something important (you may not recognize that their current actions are important), be prepared for a major ass chewing.

Yes, you can make simple, "on the spot" corrections. The NCO maybe not be aware of the violation and might appreciate your pointing it out. But...it is not your role to correct "jack up" NCOs. PVTs are still learning how to be a professional soldier and the best thing you can do us ensure that your appearance is correct.
When you do point out a correction DO NOT do it in a "smart ass" or disrespectful way. I've never seen anyone get an Article 15 for being disrespectful to PVT but I have seen it when a PVT thought he had the authority to correct an NCO. If the NCO "blows you off," it's not your role to insist that they follow your instructions, you don't have that kind of authority.
A "jack up" NCO is the command's problem to fix, not the PVTs.

After thinking about your question a bit, I feel like expending on my answer. But first, what do you mean by "jacked up?" I'm also curious why an E-8 feels compelled to always be correcting an E-2 instead of talking to that Pvt's next in line. I would tactfully (yes, it goes both ways) do an on-the-spot correction as a professional courtesy, but if the soldier showed a consistent problem with being squared away, my "correction" would include a session with his immediate supervisor in my office. If my uniform back pocket was unbuttoned without my being aware of it or my fly was open, then I would welcome someone letting me know. If you're talking about them being overweight, then leave that to their CoC. I'd prefer one of my soldiers watch my back and let me know something than have my CO or CSM call me out on it. The PV2 doesn't rate or review my NCOER, those people do...

Yes. Use tact. Show in black and white where they are wrong. Do it away from his or her peers, as correction should be done privately. However, I do suggest having your first line leader be aware.

Step 1,. Figure out his personality
Step 2,. If jackass go to your NCO and let him approach NCO to NCO
Step 3,. Watch out for anyone who commands by the rock paper rank mentality. You will lose.
Step 4,. If he is always correcting others and not squared away probably refer to step 3.

Lastly, sometimes the best NCOs become the best NCOs because you learn early what not to do. Remember, it's your career, don't waste a few years over a seniors who. Work your ass off, get promoted and move on.