For gaming purposes, decorating your shop in Recettear is completely dependent on Atmosphere in regards to how many and what type of customers you attract. The Floors, Wallpaper, Carpets and Tables along with each item you put on display are simply just a means to an end in adjusting said Atmosphere to the desired level, which is typically center grid (You can talk to Tear to find out your Atmosphere level).
Basically, there is no ideal way to decorate, it changes all the time as it forces you to adapt to compensate for the items you put on display which contribute their own points to the atmosphere score. For example, if you have too many gaudy treasures on display like Golden Scales, you'll have to offset it by using plain decorations like Plank Floors to balance it out, otherwise, it'll alienate the majority of customers.
The only times you might not want center grid Atmosphere is if you are trying to attract certain customers, such as say a certain demon who loves dark atmosphere. You can also benefit from having slightly Light/Plain during the day and slightly Dark/Gaudy at night, but center is pretty safe all around. You might also want to adjust your Atmosphere towards Light/Plain to keep a certain customer away, you'll know what I mean if you haven't met her already.
So long as you keep your store Atmosphere near center and put things in the store front windows that customers can afford and are interested in (Food, Weapons, Armor and Accessories are usually the core categories), you should get enough customers each session to reach the 128 Just Combo bonus with each time.
On an aesthetic level though, my favorite shop style is Eastern Wallpaper, Leaf Floor, School Counters and Red Carpet. It gives a nice Japanese garden feel and is pretty good in offsetting what I usually sell.
Though changing atmosphere is hardly enough to keep Customer E away. I've had her shown up regardless of what atmosphere it was; plain, gaudy, light, she shows up anyway. Granted, I do have 3 hearts with said character. At least she's useful for getting some rare treasure. I actually want her to come back more often so I can fill up my encyclopedia. She does peddle some really bizarre items as treasures...
She's not all bad, she sells some Crystal Nightmare level Treasure items near the highest customer reputation like Pitted Pots, Old Teacups, Sticky Pottery, Old Clocks, etc... Then there are the times when she sells Checkered Floors that I can just buy down the street and I want to murder her.
Having a Light/Plain atmosphere wouldn't guarantee to keep her away, but it does significantly reduce the chances of her showing up, she does occasionally sell something good though. If only she didn't sell items that you could just buy at the Market or Merchants Guild at inherently more sane prices she would be a much more welcome sight, but I suppose she wouldn't be quite as fun to hate if that were the case.
I hardly ever bothered with the decor aspect until recently. Now I just change the carpets and it's enough to get the atmosphere pretty much in the middle. I think I would have preferred it more if it was more of a way to personalize your shop instead of having to constantly alter it whenever you open the doors to customers. I know you could set it up how you want but, what you want might only get 2 customers to walk in.
I don't know if it's because I've been playing so long with the default setup but it's also my favorite. It just looks more like a shop.