Chiming in. Like you, I fluctuate between two sizes. I have clothes in both sizes, and I won't change that no matter what decluttering experts say! After all, I own those things for PRACTICAL reasons!
I tend to gain a few inches on my bust (it's awful!) mid-way through my cycle. I have tried everything - it doesn't change. Sometimes weather fluctuations also make me gain weight slightly. And I'm NOT ok with tight tops or dresses, anything that starts to press down on my bust. It's the main reason I have two sizes in my wardrobe, and I wouldn't change that for the world. When you're feeling bloated and iffy, wearing a top too tight will only exacerbate your misery (it could even be pants). I would rather be comfortable than have a tight wardrobe that just doesn't cater to my needs for those days, even if it's 3-4 days a month. Some months this fluctuation doesn't happen at all, but I'm still ok keeping those roomier and comfortable clothes. I do have 3 bras in a bigger size too btw.
As you'd have already guessed, I do not feel guilty about this at all. And I do have regular home wear tees, nicer tops, simple dresses and even dressy dresses in the larger size. Basically - I'm prepared to feel happy and well dressed in any situation even if my weight is up.