I’ll be straight up with you: I don’t use my mic anymore on pubs unless I’m with people I know. I try to take somebody with me to a TF2Center lobby if I do those nowadays because I know some motherfucker who just can’t accept that all sorts of people are playing this game, including people who should not be playing this game since they are under a mature audience, will have an issue with the pitch of somebody’s voice if it is not male unless they know they have boobs and are cisgendered is going to have an issue with me.
But using voice communication or making casual calls in pubs helps, like if there’s a Spy running around as a Pyro, if there is a sentry that hasn’t been taken down yet, or if there’s just a player that’s giving grief to the team because they’re racking up the kills and being a big problem for the push. If you’re nervous about it, pay attention to whoever else is using their mic on the server. Valve servers often don’t have communication so using the mic while you’re alive (everything while you’re alive mind you, gravetalk is disabled on Valve servers) wouldn’t hurt to try it. Don’t expect everybody to listen to you though, and expect at least one asshole who doesn’t know how to respect you whether it be your skill or your voice. Beyond one asshole, leave the server and join another one.
Also, check out the rules of servers on the !motd or when you join if there’s rules about sexism or harassment. Usually, you’re safe on those servers. If anybody is part of any gaming communities or clans who wants to pimp out their server for this lady that will welcome her for her skill level with open arms, reblog or submit your servers and I’ll post ‘em up!
My 2 cents [if you don’t mind me adding some]:
From your question are you jumping from doing offline practice to online servers, or from pubs to lobbies/mixes/comp?
For pubs, I do not think it’s necessary at all. If you don’t talk, people just won’t care. Treat them as a bunch of targets for you to practice whatever you want to do on them. It’s usually a chaotic mess anyways. An exception would be if you notice people are genuinely friendly and don’t seem to mind voice chatting. If you are worried about speaking though I’d recommend you check out TF2Newbs servers, I play on their servers 90% of the time I’m pubbing and everyone’s super chill. Sexism, rascism, slurs, etc, are usually reported quickly to admins and friends really look out for each other. [Plus I have a lot of friends who frequent server #8 and there’s a lot of ladies in our group.] If you want to play with friends but don’t want your voice going out to the rest of the server, you can use Mumble or Teamspeak.
If you’re transitioning from pubs to lobbies/mixes/teams then yes, you need to speak. Lobbies are hit-and-miss in terms of friendliness but most of the time I find them just fine. Newbie Mixes are catered to new players wanting to learn 6v6 so the environment is friendlier to help your calm your nerves a bit. As for teams, if your team treats you like trash you have every right to leave and look for another. The best part about a team is that you can meet some cool-ass people who want to play games with you.