“Purified” is a bit of a general term, so let me break apart the potential issues: 1) Distilled water shouldn’t be a problem. 2) Reverse osmosis filtered water may or may not be a problem depending on the water source. Some dissolved minerals could conceivably get through, and minerals are generally going to be ionic. 3) Ionic contamination is probably not going to be an issue for stencil printing itself. It is migration and cross contamination that you need to worry about. If you are using rosin or water soluble flux and are cleaning your boards anyway, it is probably ok to have some contamination. If you are using no-clean flux and are not cleaning your boards, it is better to stay with distilled or DI water. 4) Minerals could conceivably affect how the paste releases from the stencil surface and build up as deposits. This wouldn’t be my top concern, but a potential risk.