Of the options, I would think that a relationship between Riker and Ro was the most unlikely to work out (you know, apart from the fact that the characters were only doing it because they'd been memory-wiped and didn't normally get along), given their ranks. I would think that there might be more of a potential for abuse of privilages - the Commander giving the Ensign a promotion or two to give them better job opportunities and such. I suppose it could be done with a Captain and Lieutenant Commander, like Picard and Daren, but a Lt. Commander has had more of a chance to get to a job they want, and if the Captain were to put them in for a promotion, they'd probably have to get transferred off the ship anyway.
Ultimately, though, I don't think Starfleet really has a policy on interpersonal relationships up until they interfere with the job - going to the rescue of your lover in the middle of a crisis and abandoning your post is definitely an offense, as we learned in 'Change of Heart,' when Sisko told Worf that by saving Dax at the expense of the Cardassian spy, he'd probably never get a command of his own. Starfleet was at war at the time, and so they couldn't exactly drum someone with such experience out, but had it been a time of peace and Worf had done something similar, I doubt he'd get off so easily. In fact, personally, I think it would have been better if instead of killing Jadzia, they'd had Starfleet reassign Dax, perhaps even giving her a field promotion and command of a ship elsewhere to show the consequences of Worf's actions.
Ultimately, though, I don't think Starfleet really has a policy on interpersonal relationships up until they interfere with the job - going to the rescue of your lover in the middle of a crisis and abandoning your post is definitely an offense, as we learned in 'Change of Heart,' when Sisko told Worf that by saving Dax at the expense of the Cardassian spy, he'd probably never get a command of his own. Starfleet was at war at the time, and so they couldn't exactly drum someone with such experience out, but had it been a time of peace and Worf had done something similar, I doubt he'd get off so easily. In fact, personally, I think it would have been better if instead of killing Jadzia, they'd had Starfleet reassign Dax, perhaps even giving her a field promotion and command of a ship elsewhere to show the consequences of Worf's actions.