Well, I've never seen "LF melee dps <insert operation name>" in general chat. "LF rDPS" ? Many times.
But I don't pug operations (only within guild.), so I really can't tell what other ppl prefer. In my guild there are more ranged dps than melee ones, but just because that's what people like to play. There are no "requirements" in that regard.
I see LF rDPS all the time. I also say it a lot of the time. Why? Because close to 75% of the DPS that whisper me are Sentinels if I just say LF DPS. Most of the time thats not an issue, but when I'm pugging LI HM, ranged just makes life easier. I only run melee through it when its guildies on Vent so we can get the in/out timing right or if its a melee I know. It's also because a lot of people won't run LI HM with melee and as such, they all jump on the chance to get in and finish the quests if you just say LF DPS.
In terms of ranged vs melee for Ops: I like to take 2/2. I've run EV with 4 melee before and its easy enough but on EC you're making life hard by taking more than 2 melee DPS. KP can be done with 3 melee without too many issues but 4 is real tough.