It entirely is up to you honestly. I have seen many impressive painters not prime at all based on the sole fact that the environment which they live gives a very small window to use sprays effectively. Places such as Texas, and other very dry or hot climates makes priming with sprays a bit of a pain.
That said, if you do choose to not prime your models, I would suggest giving them a bit of a cleaning before assembly, just so that you don't have any leftover oils and such from the molds.
It can also help to seal your models with a matte varnish (or gloss, satin, etc whichever finish you are looking for on your model) to give the paint a bit of protection to help it stay on the model.
Essentially, primer is just a layer of paint with special properties that helps the paint stay on the model, and is not always necessary.
Always prime your models if you can, whether its through spraying or undercoating in one solid colour do it. This will give the paint something to stick to properlywhen it gets so layering without it rubbing off easily, which can happen when you paint in thin layers.
Personally what colour I choose to prime in depends on the colours one top, dirtier models I go black, brighter models I go white, or if there is a specific spray that fits like the GW colour ones then i go for those.
I prime everything. I ran some tests to see how paint sticks to models with and without primer. Some I even primed it so half the model had primer and the other half didn't. What I found is that on plastic, it didn't really matter (IMO), but on metal? Prime prime prime! I've had so much trouble on metal that did not have a solidly good primer coat that I've had to strip entire batch's of models to start over. Most recently was 30 mechs I bought for Battletech. Doing a camo theme on them, I used tape to get it set up the way I wanted and every time I pulled the tape off of a layer, the one below it came off. Once I stripped them and made sure the prime coat was perfect, I had no problems.
So, in short, I recommend priming.
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