It’s nice that I can choose metric units like mm for the ruler. It would be more convenient if the ruler showed round numbers of the chosen unit and the grid had follow it!
This doesn’t sound like a question rather like an enhancement request - If so, please file an enhancement request at LibreOffice Bugzilla
If you change to non-ISO centimetre rather than millimetres your wish will be granted. Tools > Options > LibreOffice Draw > General > Unit of measurement and change to centimetre. Adjust grid to 1 cm and have margin of 1 cm. Cheers, Al