Yes, this is true. The reason why they are stronger is because you are infusing it with your own energies. When you make your own spell or ritual, try to make it along the lines of others. e.g: Dont decide to rap instead of chant because thats disrespectful and wont work as well.
They have to be created from your heart. Yes, you could just make it up on the spot, but once again, it id more powerful if you spend your time thinking it out.
This is a very useful website to use: http://www.llewellyn.com/journal/article/1864
I hope this has helped. Blessed be! )O(