Should levels in games always get harder?

I have been working hard on the levels for Mighty Marbles and I have got to the point I have completed enough to start ordering them. I went in with the just make fun levels and worry about ordering them later.

So this has left me thinking what is best practice? Do you go easiest to hardest? Do you go in more of a wave where they mixed up so people get a break with a more relaxing level. Does it really not matter much at all? Do you base on how long they take to finish?

So curious to hear everyone's strategy with find the right order for levels. Currently mine is generally get more difficult but including some easier ones later on so people can show their mastery.