In my experience, -たち, is used when it's necessary to emphasize plurality. I've also seen -ら used the same way, but only in familiar circles. That's not to say that it would be insulting to use -ら for people with whom you are not acquainted, but that's just what I've observed.
An example is when I was on my exchange program in Japan. We were getting our picture taken at 白川郷. All us students were ready to get our picture taken, and then the guy taking the picture said "先生も！" Only the one professor came up to get her picture taken so the guy says "先生たちきてよ！" and so they all came up. So that's an example of a situation where it's necessary to clarify plurality.
Additional, I've only seen -たち and -ら used for a group of people, such as 私たち, 僕たち、先生たち, and even 子供たち. I've also seen あなたら,僕ら (us, I've heard that in songs a lot), おまえら, and 彼ら (they, them). So generally I only use it for that specific set of words. That being said, I would say that -ら itself isn't deprecating or informal, but rather that it is only applied to deprecating or informal pronouns.
Hope this helps!