mmm, rule of the thumb is that the more power your appliance require, the bigger the difference will be between 100V and 120V. hair dryer is a prime example of a power thirsty product.
Panasonic Yeast Pro Automatic Bread Maker -- ON SALE FOR A LIMITED TIME!
the above bread maker for instance requires 550W, which is quiet a lot, about half of a normal hair dryer (1,100-1,3000). unfortunately i can't give you an exact answer whether or not it will require a converter, but if u do decide to buy one make sure that the converter will be able to handle what ever power required by your bread maker. a converter covering 550W is usually rather big and heavy, and is usually as far as i know not found in a regular electronics shop.
here is a site in the US,
Power Converter - Step Up/Down Japan Transformers