Having spare pumping capacity adds some capital and running cost but does tend to be more tolerant of gas-load variations. Systems that are run at the limit may be more sensitive to variations than those that have some built-in tolerance or spare capacity, making the process more reproducible.
The aluminum-evaporation process aims to have a source-to-substrate distance that is shorter than the mean free path so that a minimal number of depositing atoms undergo any scattering collision. This benefits the coating by also having fewer gas atoms incorporated in the coating, making the coating slightly denser. A higher optical density for any given thickness of coating is possible rather than a coating produced at a higher pressure. The coating also will have a higher reflectivity and surface smoothness compared to coating deposited at a higher pressure with a greater amount of scattering of depositing atoms. In addition, the deposition process will have a slightly higher material efficiency. So, to some extent, the rule of thumb that I was first given could be applied in this case, and that is: The lower the vacuum the better the coating.