For Servants? No need, balancing will be inherent as per TM mechanics. Parameters will just have to suit the Heroic Spirit in question. A Caster with B rank STR for instance is unlikely to happen, though there are some exceptions depending on where you look.

As I said, we're operating on TM mechanics, so TM canon will be used as a baseline for judging the plausibility of something like stats. If someone tries to app Hannibal with A rank Mana or Volundr with A rank Agility, that'll be pretty substantively implausible as per their own legends/histories and comparison to canon Servants.