I have had some bolters, say 5% from March sown leeks, in modules in the greenhouse. I find that all sowings before about mid April carry higher risk of boilting because leeks are biennial and then think they have perhaps been through a winter, so flowering tendency is increased. Outdoor leek sowings are best made in the middle of April in my experience.
Onion sets planted before about mid March carry similar risk, I find early April is good.