For general riding, I find that 28 are best for good roads, 32 are best for bad roads and light trails, OR for loaded use on good roads. 38+ are good for loaded use on bad roads/trails.
Ive done a lot of riding with full loads, rough trails using 32mm. It can be done but a wider tyre is preferable for better shock absorbancy, grip and control.
32 are my usual 700c tyres for general purpose riding.