When this particular question is asked, you are bound to get different answers. Most of the time the answer would very much depend on the bra size of the individual. As they say: different stokes for different folks. I did a little research on this interesting subject and got different opinions:

It really depends where you live. In Europe, it's common you may proudly walk braless in our natural innocent curves if you have firm and fit (not saggy) breasts and may be proud of them.

Just out for a walk in the city, around the house, maybe in a local market, etc.?
And no, I'm not a 70 year old woman, I'm 20 but I have small breasts (US 30 A...UK 30 B... Europe 65 B) so I don't know if I should go without a bra?

With smaller breasts you really should be okay. Any bigger than what you are would be pushing it though. If you are wearing an extremely thin shirt and your nipples easily poke through, then you should wear a bra.

When I was a 38DDD, I would not step out of the house without a bra. Now that I'm a 38C I'll go almost anywhere without one, but for modesty's sake only if the weather allows me to wear a heavier garment.

There are times we have to consider other people. This is something you can decide for yourself. Do whatever makes you more comfortable.

Well it's up to you really, as the other person says you would probably get mixed reviews from others, but in my opinion it's nobody's business (except maybe at work where it may convey the wrong image to customers/clients).

The above views in my opinion make good sense and should be considered when going out. At the end of the day, I believe it depends on where you go, what outfit you're wearing, what size ladies bras you wear and what you feel comfortable in. Of course you have to take others into consideration especially if you're in the midst of people you're not familiar with.

Many women tend to go without ladies bras, not just because of their bra size but the discomfort they seem to experience with wearing a bra. This can easily be solved by wearing your right size by going for a bra fitting aka bra measuring by a professional Bra Fitter. If this is not possible, you can easily do your bra measuring online. There are a lot of Benefits of wearing a properly fitted bra especially if you wear above a C cup.

At the end of the day, one has to apply commonsense and sensitivity to others. However, the final choice lies with the wearer.