Let's see, I think planning to have answers ahead of time is great!!!
When at a bar: "I'm the designated hair holder for my friends when they are later kneeling before the porcelain" "I'm driving" "Just not feeling the booze tonight" "Why do you ask?" - I would keep these answers light and funny, gynecological issues aren't really bar topics, (unless it is informing another person about an STD when planning to be intimate). I don't think you would want to hear about a fella's jock itch when sipping a cosmo

Going out to a bar is a time for fun, so it probably won't come up in conversation unless you bring it up.
On a first date: Unless you think he is about to propose on this first date, you don't need to discuss it. Who knows if you will even have a 2nd date, and probably won't if you are talking about gynecological issues... what would you think if he was talking to you about something related to his testicles on a first date? If you start getting to date #15 and are talking about the future together and the discussion about kids comes up, then it is appropriate

If he is questioning you about your food choices and not drinking (instead of getting to know you and enjoying your company) I would be a bit concerned, what business is it of his what you eat or drink, get my drift?
At a new job: again, keep the gynecological issues out of the picture. Unless, you have some gnarly complications that will cause you to miss work and then you need to be able to provide documentation from your doctor about reasonable accommodations that you will need. This gets to be a lot of paperwork, but if you need accommodation for PCOS, make sure you get it. No need to mention anything health related in an interview and no need to talk to your co-workers about it around the water cooler, it truly is none of their business.
When you are feeling cruddy and need to remove yourself, just simply say "excuse me, I need to go lay down for a bit, I'm not feeling well." Nothing more is needed

Your close friends already will know, so no need to explain. If you are in other situations, it truly is no one's business. For example, you are at work, let your supervisor know you aren't feeling well and need to clock out for a bit, but you will be back. Employer's can not ask questions about health, not legal, but that doesn't mean you are excluded from following any company policies. If this really is an issue that comes up frequently at your job, you need to speak with your HR department for your protection. It's a simple chat, so don't stress yourself about it - smile! Also, if you don't have an understanding employer or supervisor, you might think of finding a better job. Work supports life, life does not support work! Meaning you don't have to give you mind, body and soul to your employer, those parts are for you, family & friends, work just pays the bills so you can enjoy your family & friends. This is a fine line to walk when you are trying to get ahead and establish your career.
Some days I feel like I have a neon sign on my head flashing PCOS. I want to tell everyone, but it isn't always appropriate conversation. Not to say that you should never mention it, but the time will naturally arise when it is right to discuss. and you always have the right to say "I would rather not discuss that right now, where did we all want to go to dinner/movie/mountain climbing?" A little bit of mystery can do a girl some good, no need to be an open book.