I am slightly confused by your initial post.
"Normally" a sailor only fills out a dream sheet once, during prototype. After that initial set of orders to your first sea command, when you go to negotiate for orders it is just that, a negotiation between your detailer and you.
Actually, it is much less of a negotiation now with the use of JASS. What you see are the open billets, by paygrade, available in the fleet. Period. It has nothing to do with ratios. It has everything to do with maintaining the manning aboard all operational vessels as close to 100% as reasonably possible.
Since you asked about both sub and surface, I am going to go out on a limb and conclude that you are in prototype and are having to fill out your initial "dream sheet". If that assumption (I know, assume

) is incorrect, a little more background information would allow us, your peers, to provide you with more detailed and pertinent information.
On a side note, if you can't handle the rough edges associated with being a nuke, meaning some of the responses on here, your time in the Nuke power program (and life in general) may be somewhat challenging.
Best of luck.