I've said this 100 times before, but I'll say it again here,
If you're ordering a custom and you have dark hair, then its best to order single strand, single knotted hair at the hairline.
This means that you won't get more than one hair knotted (or double knotted) and this decreases the 'dotty' look. Its also easier to bleach single strand, single knotted hair.

Personally I would specify this type of ventilation even if I was ordering blonde hair and then proposing to dye it.
Its just a better replication of natural growing hair.