Don't play with the multiclass XP penalty at all. Other than preventing crazy munchin builds it is a useless rule. Besides, it just makes crazy munchin builds more crazy/silly and more difficult while not actually preventing them. D&D 3.5 took away many of the front loaded bonuses, such as the Paladin and Ranger, so the multiclass XP penalty doesn't really serve much purpose. 3rd edition is very flexible with feats and skills to allow any character concept but part of many character concepts requires multiclassing. Why give out XP penalties on good character concepts?