Local wireless battles do not alter your Pokémon's levels according to the rules in place.
Here are the local wireless mode rules for Double and Single battles -
NO RESTRICTIONS-
Any Pokémon may enter, regardless of species and level.
Having more than 1 of the same hold item and/or Pokémon is allowed.
You may enter with a minimum of 1, to a maximum of 6 Pokémon.
STANDARD Cup-
All Pokémon must be Lv. 50 or less.
Certain Ubers are banned.
You are not allowed to have more than 1 of the same Pokémon or hold item.
You can only enter with 3 Pokémon in your party.
FANCY Cup-
All Pokémon must be Lv. 30 or less, and their combined levels cannot exceed 80.
Certain Ubers are banned, as well as Evolved Pokémon.
You are not allowed to have more than 1 of the same Pokémon or hold item.
You can only enter with 3 Pokémon in your party.
LITTLE Cup-
All Pokémon must be Lv. 5 or less.
Certain Ubers are banned, as well as Evolved Pokémon.
You are not allowed to have more than 1 of the same Pokémon or hold item.
You can only enter with 3 Pokémon in your party.
LIGHT Cup-
All Pokémon must be Lv. 50 or less.
Certain Ubers are banned, as well as Evolved Pokémon.
You are not allowed to have more than 1 of the same Pokémon or hold item.
You can only enter with 3 Pokémon in your party.
There is a maximum weight limit of 218 lbs.
DOUBLE Cup-
All Pokémon must be Lv. 50 or less.
Certain Ubers are banned.
You are not allowed to have more than 1 of the same Pokémon or hold item.
You can only enter with 4 Pokémon in your party.
From this, you should be able to tell what the ideal levels are :P
Personally, I would be using the no restrictions rule, and use my fully trained Lv. 100 Pokémon. If you level a Pokémon up to even 51, then it will be unusable for all of these rules except "no restrictions".