Best to ask your assembler, as it will depend on their capabilities and workflow, as well as your intended quantity and the size and assembly needs of your PCB. Fabricating a super thin board is one thing, assembling components onto it is another. Supporting the board during stenciling and placement will be more important the thinner and larger the individual boards are. Some designs may require extra setup or even custom support fixtures, which could be very expensive if you're only building 5 units, or a negligible cost if you're building 5000.