Insulation, up to a point (thinking hugely excessive smounts) is good all year round. I never remove any, except that which I used to strap to the sides of some hives.
Bees are perfectly able to regulate their hive environment at a suitable temperature as long as sufficient ventilation (all at the bottom of the nest, none at the top) and water can be sourced in times of excessive temperature.
Insulation retains heat during the winter months, allowing the bees to maintain a safe cluster temperature with minimal stores consumption, and reduces direct heating (due to insolation) to a minimum during the summer. Win win, as I see it. I doubt anyone will argue the above, but go ahead if you must.
I used celotex like Phil - only I just used tight fitting slabs with a foam strip to seal against the crownboard, to reduce heat loss over the top. I was not bothered with sealing at the bottom of the box (I over-wintered my bees on only one box, generally).