If you're going NROTC there is a 0% chance they will an allow an educational deferment or select you straight out of undergrad for dental school. This used to happen occasionally to top candidates, however NROTC no longer does this. If you have the grades to go to dental school and haven't taken the ASTB, you're likely to find yourself operating a nuclear reactor.
I'm a former NROTC instructor and watched students with these aspirations at a top university with grades and aptitude for any medical school in the country get denied due to the needs of the Navy every single time. Do not go into NROTC without understanding you WILL be going into an unrestricted line community until your service obligation for NROTC is complete.
That being said... if you want a good path to being a Navy dentist: go to undergrad and keep your student loans to a minimum at a good public school, and then apply for the Navy HPSP / HSCP at the same time you are applying to dental school. Or, do what I did, do NROTC, serve your time as an Unrestricted Line Officer, and then apply to dental school.