Levelling up creatures

Hi, some time ago I saw under some other post something like you shouldn't level up your creatures to the MAX level because then you have hard time progressing in Beyond in Magizoologist career or something? Not sure if I remember correctly, but that's why I'd like to ask someone to clarify this for me and tell me if it's better not to level up the creatures. I'm def going to level up those with SQs where you need it, but for example I've just finished the Chinese Fireball SQ and the requirement was just level 6, so now I'm unsure whether go on levelling the dragon up or not. Thanks for advice in advance!