I definitely dont take Death scales, thats for sure! Growth is nice but not required. Generally I just ignore old age. If a mage ever gets Diseased I just teleport him to the front lines to get some use out of him before he dies.
With pythium i always try to take good scales including good growth since pythiums real strength comes into play i mainly plan for then(+good scales means large, well supplied expansion armies) but i also try to get 4 nature on my pretender. The nature magic on the pretender not only helps with magic versatility but it also gives a minor regeneration bless that stops units taking damage from disease if you give them a shroud of the battlesaint(5 pearls and fairly low construction iirc). It also means you dont HAVE to give your angel thugs/SCs regeneration if your just using it to avoid the occasional affliction(if you expect them to take a fair amount of damage they need either more regeneration or a new build).