It depends what you mean by sagging. Some girls are very lucky and have firm breasts made of quite bouncy tissue, and they can't hold a pencil underneath. I've never been very big, but mine have always sort of hung down a bit rather than being up and perky. On the other hand, they still look exactly like they did 20 years ago, so it isn't "sagging", really, it's just the way they are shaped. The trouble is the pictures one sees in the magazines are only of the big perky ones! So it makes you feel inferior. I couldn't believe when I joined a gym about 15 years ago. The changing rooms were just open, and it was the first chance i'd ever had to see what ordinary women look like. The variety of breast size and shape is incredible.
But at 34c it's a bit unrealistic to expect to go braless in a t-shirt, anyway. That is the way you would end up sagging one of these days.