First I think it depends on how much you fluctuate. Is it a TTOM fluctuation? Then yeah make room in your closet for clothes you wear once a month. Next consider how much weight creep does it take to move up a size? How many pounds does it take to move up a size? And how long for that weight to creep up? A year? Two? Longer? Consider fashion cycles.
Example size 0-2-4 takes only 2 lbs between sizes, whereas 4-6-8 is more like 5 lbs, and 8-10-12 is more like 10 lbs, 12-14-16 is roughly 20 lbs between each size -- this isn't about clothing manufacturers being dumb, it's about the paper towel theory. And, it can take just as long to lose those "last" 2 lbs as it can take to lose the first 10 or 20.
For years I maintained my weight within +/- 5 lbs and being on the smaller/shorter end that could translate into anywhere from a 0-2-edging-into-a-4. So I had a mix of those sizes at all times in my closet. Then my weight crept up into the solidly 4's, and I stayed there for quite awhile, because I had 5 lbs to work with, not just a measly 2. Until last couple of years with health / new job / renovation etc etc... and it crept up again into the 4-6 range and then into 8's. At that time I banished all too-tight clothing from my closet -- but packed it all away, as I was figuring the weight gain was temporary due to lifestyle stuff.
It's still packed away but lifestyle has settled out (health issue solved, and renovation is over and we have a working kitchen once again!) so I am now on the weight loss wagon! In a few more weeks / another month I'm expecting to be able to start unpacking some 4's and eventually maybe even some 2's I've saved. I don't hold out aspirations to get back into any 0's again. I was comfortable and happy in my 4's, so that's my goal.
I think it's critically important that whatever you save should be fairly classic and not too trendy, and be items that were hard to find in the first place. In good condition. That's the sort of stuff I packed away. When I unpack them again I'll go through it all and likely do another ruthless declutter. Then again, I am missing some key items and can't wait to break them out, and I'm happy knowing they are tucked away in storage right now waiting for me. And that is keeping me from going too crazy buying new stuff right now -- while motivating me to stick with the diet since I know I have perfectly good stuff waiting for me once I drop another two pounds.
Knowing I was calorie counting I did have trouble the last few months choosing between the larger (just right) and the smaller size (slightly too tight). I went with the just right. For trendy pieces! That will go out of style next year anyway. For more expensive pieces I just tell myself to wait awhile longer. Exception is blazers, I bought three new ones last month. Because my shoulders are the same regardless of my weight; the blazer just hangs better.
I'm writing a novel here but one final point: I do consider buying new clothing as a reward when I reach weight loss goals. That's stopped me more than a few times. Definitely do not get anything hemmed until you reach your goal: many items I own have gotten longer, as I shed weight. I try to evaluate any new purchase right now with the question whether it will still fit me when I drop X pounds. Tops are more likely to pass that test than bottoms, because I lose weight first in my lower half.