It depends a bit on the type and size of the paper. RC/PE paper stays a bit flatter than FB paper. With an 80mm enlarger lens at f8 you'll have about 1cm depth of field on the paper. But I think that it is better to use an easel to keep the paper almost perfectly flat. It will give you maximum sharpness. A smaller size paper that lays loose on the enlarger table also tends to curl during the exposure which will cause blur.