I agree with everyone else - typically, I wear lower cut, v-neck or "stretchy" neck tops, so that I can easily pull a boob out of the top. In the summer, that looks like tank tops. In chillier months, it can be a v-neck long sleeve shirt with a cardigan sweater on top. If it's really cold, I can even just unzip a coat a little ways, keep my layers on, and still whip out a boob. If you have a scarf on - even easier - you can use it to cover your babe and your boob!
Layers come in handy in a variety of ways - wearing a light blouse, button up sweaters or open front jackets on top lets me pull one side over to be a quick curtain around DS, if I ever want to cover up (though by 2 years of nursing, those incidents are few and far between! ) or block DS from wind, chill, shade him from sun, etc. This is probably easier with a child who can sit up, so I have that extra hand free, but it's super convenient. Much better than digging out and fumbling with a cover, and easier to put down/pull up.
Basically, I don't often wear crew-neck tops very often. Almost all my shirts/dresses are v-neck, wrap-style, scoop neck, or simply have enough stretch in the fabric to pull the collar down. I don't wear many sweaters or sweatshirts UNLESS they are v-neck or button/zip down the front for easy access. But that's just me - I'm more of an "over" than "under" gal (especially when I was self-conscious of my post-partum belly showing).