Large seeds are probably better sowed directly where you want the plants to grow.

Very tiny seeds can be difficult because of the possibility of drying out. So it depends how diligent you can be, and how organised. Are you going to remember where you sowed things and where you need to keep the area moist? Or are you going to be like me, wandering around the garden in a vague way, trusting to memory, getting side-tracked. And wondering why the ___________ [fill in blank] never germinated.

Another thing you have to watch out for is the frustrating cutworm. He is a caterpillar, in his larval stage, and will become a moth. But on the way to getting his wings he spends his time slinking about chomping through the stems of the very plants that you have sown. Like cucumber seedlings. You will see them - both the felled seedlings and the caterpillar - but you won't suspect the caterpillar because of his air of innocence. And he's so smooth and attractive. (You probably only see them though if you've got your hands in the soil, fiddling about.)

So, you need to protect the seedlings of things like zucchini, pumpkin, cucumber - all those nice fleshy seedlings. Little seedlings possibly aren't attractive to cutworm. (You can use something simple like a plastic milk bottle with its base removed, and no lid.)

So, in summary, the main thing is to keep the area moist. And to know how deep to plant seeds. Little ones are planted very shallow. Big ones, about .5 to 1 inch. Don't walk on them. Don't stand on them admiring the garden. And there is just one exception when it comes to watering: with peas or beans, water at sowing but not again until they have germinated, or they will rot.