Maximum SPI clock speed specified in data sheet
No, 6 MHz is guaranteed to work by the specification.

In practice you will very likely find devices which can work up to 8 or even 10 MHz. But it is not guaranteed that that will always work for all power supply voltages, temperatures and all chips from any batch.

The 6 MHz however is guaranteed over all conditions. So no, you do not need a margin as long as your 6 MHz is accurate enough (and it probably will be as it is usually derived from a crystal based clock).