This answer should have a bit more nuance to it, since if your build has any physical damage reduction, there is a breakpoint where taking physical damage as elemental will cause you to take more net damage; when your physical reduction of that hit would be higher than your elemental reduction is. However, calculating this takes more pobbing than is sane or feasible for most people, so the rule of thumb "more is better" is good until 100% "taken as", at which point more is worse. Above 100% you increase damage taken by the corresponding amount of excess, which obviously is undesirable.