I'm having a new fence installed and was curious about if I should stain/seal it.

All posts and rails are pressure treated pine and, the slats are cedar.

Not needed on those materials

All depends on how much time and money you want to spend for what result.

You don't technically need to do it but those materials are going to gray in time.

I recently had a new fence installed with those materials and used Ready Seal. It was a bit of work as I used a foam pad since my neighbor on the other side wasn't staining theirs and the spray tended to go through any knots so they would have splotches. Didn't want to be that neighbor that messed up their fence.

It was quite a bit of work but I found the effort rewarding as it looks much better. Not sure it's going to extend the life of the boards that much but appearance wise it stands out.

I lean towards staining my new fences with an oil based stain. The color lasts longer and I have had fences last longer.

Keep some on hand to replace the random picket/board that will eventually break (actually stain a few extra too to keep as spares).

This 100 times. They changed the use of the old style chemicals used for treating lumber a few years back due to regulations…. And the treated lumber today just does not last like it used to.
For this reason it's a good idea to treat the lumber with a quality oil base stain/preservative.
We use a product called TWP on our horse stall lumber and log home and it holds up really well. Recommend by the folks who built the home.
Not cheap but really works. Comes in all stain colors or natural clear.

Do you mind me asking who you used? I was planning on going with Texas Fence, but they decreased the discount for doing multiple sections. They were $27.50/LF for a three rail with a 6" rot board, but it's now $30.50/LF. That seems pretty high to me.

https://www.homeadvisor.com/rated.3BrainsConstruction.109121220.html

Talk to Jaime

Brand new treated lumber won't absorb paint or stain well, so you will need to let the fence 'dry out' first for a few months.

IMO stain looks better, but will not protect as good as paint will. I stained my fence a few years ago and I wish I would have painted it. I did two applications of stain, spent about 1100$ on said stain, and now 3 years later it looks like I need to do it again.

Get paint, coat it thick, and forget about it for a while.