To my way of thinkin' (changes when the wind blows), if you have a 40, then you probably don't need a 36, but if you have a 45, then 36 makes sense.
If you have a 50 or 54 then leave yourself some room if dropping down to 30's so you can has a good excuse for a 40-somthing.
Or like this 3x, 4x, 5x-one of each, then start "splitting the middles" or playing with bigger/teenier bores or smoothies, doubles, swivel-breeches, pistols, canons...
Pretty sure i need one of everything (and two of the interesting ones___this may take a while).