I am not too familiar with the logistics of spell casting, but I do think that the full moon is a great power source. It is very possible that spells cast during the full moon will produce a more amplified effect. It is not necessarily more powerful than ones cast during a non full moon night, but it could have an additional boosts. All of Ashra's spells are powerful and are designed to have similar potency no matter when they are cast (hence how she is able to handle so many requests). I think the moon does matter though, if you do not have as much experience as Ashra and you needed an extra magical connection to make your own spell casting powerful.