I have a 90 degree fitting coming down from my shower valve. Also have a screw on tub spout trim. Right now there is a 6" galvanized pipe that connects the 90 to the tub spout trim. Is the galvanized steel the best practice for this (or even code)? If galvanized steel is bad let me know how to fix it before I grout and caulk that area.
Galvanized pipe is often used when a threaded pipe is required, since it's a bit more durable than copper. Copper is a softer metal, and may not hold up as well against the potential abuse the spout could see. While it's true that galvanized pipe has corrosion problems, such a short length where it's not common for water to rest should not cause problems.
If you're using a spout that doesn't connect using threads, you may want to use copper instead. For a threaded spout, however, galvanized may be the better choice.
I try to replace galvanized when ever possible. I've noticed in tub/shower installs water eventually tends to come out discolored due to rusting of the galvanized piping. Also, I've found that galvanized, when installed in direct contact to copper or brass tends to rust quicker and I've been told by at least 2 different plumbers that it is because of the chemical make up of the galvanized piping.
Galvanized pipes were used pre-60's. So it was to code for its time but its the worst piping to use for water. Hot water lines clog up a lot with galvanized if you have hard water. Some people use PEX tubing to replace it because its remodel friendly due to its flexibility but I've have no experience with that. Unless its clogged up I wouldn't mess with it. +10 for good old copper tubing and its antimicrobial properties.
Though I can't attest to its long term health issues or your local code practices, as far as durability goes for this particular application you won't have trouble using galvanized pipe feeding your shower head or the tub spout. Typically the faucet will fail and be in need of replacement before the pipe fails. Seen these in hard water and long time soft water users. Never seen one clog or rot before the faucet became irreparable.
I have galvanized in my commercial building only for feeding fixtures such as toilets and sinks. I am replacing what was installed in 1947, much of it was ready to fail but it still lasted almost 70 years and yes the fixtures and shutoff valves also needed replaced. Brass is way too expensive to justify but for a drinking fountain, use copper.
I am working on commercial properties and I just had to replace plumbing with a galvanized nipple in copper. Water DOES sit in that pipe. The pipe doesn't have to be full or under pressure for corrosion to happen.
I found that the problem with corrosion in that pipe isn't closing the pipe off, it's corroding the pipe and fittings so that when you try to unscrew the tub spout, the copper fitting breaks before the corroded threads break free.
Break out the saws all and replace copper in the wall... Or just start with brass.
I am now going with copper or brass rob spouts.