Daystrom Institute

They are often up, but not at full power. TOS made many references to the computer raising the automatic defence screens, and by TNG, starships had navigational deflector shields that were always on.

1. I don't know for certain, but it seems possible that shields may not be usable at warp. I don't remember any specific episode where that happened, but it seems possible. But even then, a ship could just be programmed to bring them up as it drops out of warp.

I want to say it's the opposite, actually. Shields, at least on some level, are required for safe warp drive operation. Voyager had an episode where they were unable to jump to warp speed because their shields were inoperative.

However, they're probably not at full, since warp drive puts considerable strain on the power systems, enough that your average starship may not be able to power their shields at full, and maintain warp drive at the same time. You may need a high-power ship like the Protostar with its dual core system to make it work.

1. One reason brought up frequently is that raising shields could be taken as an act of aggression. But if you arrive with shields already up, then you're not doing anything aggressive, you just arrived that way, so I don't think this makes much sense in a world where most Starfleet ships just keep their shields up.

Only if they don't have a baseline. Starfleet ships seem to have no problem detecting ships charging their shields and weapons, even if it is an alien ship with technologies and configuration that they have never encountered before. There is no reason to think that aliens would be any different in that regard.

1. I guess it could be possible that the power usage of the shields is too much for the day-to-day use. But again, it seems like a lot of missions clearly begin with "dropping out of warp into an unfamiliar area" and those are the times where your shields should just be up by default.

Shields interfere with a lot of systems, not just the transporter. It also messes with sensors, which is why ships like the constitution-class Enterprise and Phoenix have their shields set up so that they're lowered on a cycle to allow the (high-power) sensors to operate in that gap.

If you're in a new place taking sensor readings, having shields would unnecessarily hamper your readings, so you may end up spending longer there, or be unable to take some readings at all.

My personal explanation is that it is a power saving measure. I got the idea from a fan made Star Trek game called super Star Trek. One could move around with shield up all the time but you’d use up tons of energy

My other theory is that it has something to do with the deflector shields.

obrien is spending all day every day fixing shit up you wanna give him more work?

Perhaps shielded vessels are more easily detected on long range sensors. If you're constantly using shields, you're continuously broadcasting your location to everyone in the sector.

Could be that the wear on the emitters would be significant enough to make it bothersome?

For 2. I feel also like arriving with shields up and then bringing them down is a strong message too, when communicating peaceful intent. Familiar races would also come to recognize that this is how starfleet does things and not consider the default aggressive.