you can stockpile if it'll make you feel better, but I wouldn't deem it necessary. personally, I get lucky with the themes she gives me, usually. makes me thankful I like so many different clothing items in the game. :^)
It's really up to you if you want to stock pile. I personally am, but that's just because I tend to have bad luck when it comes to those kinds of things, also I'd rather not try and trade someone. But I'd say the better reason to buy at least one piece of clothing a day is to get the mannequin. There's four in total.
If it reassures you to stockpile then you should, but after you pick up a piece of clothes it registers to your catalog so you can reorder it if you get rid of the item. I usually register clothes to my catalog and then get rid of them to save storage space.
I do in my second town. I bought a bunch of clothes over a few days and then have them all displayed in my museum. So if she ever appears I can just run there to get changed.
When I did my fashion checks I just get out of the game without saving, then go back in. I think she changed the theme. But I suggest you just order stuff from people online. They will usually help you out quickly and fulfill your order. Hope this helps!
I wouldn't say to "stockpile" but buy a few piece of clothing that you might need for each category and then keep them in the museum that way you don't have to wait to order them. You might get something that you didn't realize that you needed until it is too late. And yes you can get clothes from other people online but I am one to be more self reliant rather then relying on others.
Just my 2 cents.
I tried to get an outfit on the forums the day gracie came by but no one was available to trade. I suggest you order some clothes before hand and keep them in your plaza/museum in case no one is able to help you. I have sets of at least 5 different styles (I only really buy 3-4 pieces of the style and for the rest I make sure they don't clash with the style) and I've gotten lucky with 3 of them but the others I didn't have the right clothes so I had to pass :/ still waiting to complete 1 moreeee.
Something I found really helpful when I had to go through Gracie's insane fashion check was having an alternate character- you can play as that character on the same day, and she'll offer them a different theme. So if you don't have enough of the right clothes for the theme she demands at first, maybe try that? Hope this helps!!
I started ordering outfits once the TIY was built. Dress mannequin and place the themes in rows in the museum.
It's best to add accent piece from another theme that doesn't clash with the theme Gracie gave you. Do that and you'll win furniture.
For rock n roll I wore, checkerboard skirt, top, triangle shades and lace-up boots. Added ornate fishnet stocking and won a piece of furniture.