Although it sounds good not to mow,
to help the insect life, just bear in mind that if you want a nice, knitted together lawn then leaving it to tuft up and grow long isn't ideal. During the spring is when the most the grass will grow... it is the best time to keep cutting and making the roots knit up for a more dense, fuller lawn. Try to encourage insects another way...plant flowering trees, shrubs and herbaceous.
Also remember that if you let the grass go to seed it will end up in your borders and might be an issue later!