So, in my opinion it depends on the place you hunt.
If you hunt in a place where there is an obvious greater amount of a creature over the others in the cave (for example first floor of flimsy lost souls, there are like 98% flimsy and 2% grim reapers), I would go for parry. Actually I am saving charm points to buy it in my 250 ED, because I hunt mostly Bursters Spectre in Darashia.
The HP of the creature you are trying to kill, is also something to considerer: For example Asura mirror respawn: there are mostly 2 creatures, dawnfire and midnight asuras; so you could think to put an attack rune in each one (wound and poison for example); but as they have low Hp, the hit that these charms will deal to them are so low, that won't make a big difference on your exp/h. On the other hand you could use, low blow in one of them, and parry in the other one for example; or if you team hunt, your parter could use low blow in the other asura.
However if you hunt creatures with a lot of Hp and weakness for some element, go for a single attack charm, the more Hp the creature have, more it will lose when it gets active. Roshamuul, ferumbras, library are good examples where these charm runes are really useful.
Hope I made myself clear xd