For the best success with the matching method, use GIF images and exact HEX colors. JPEG images can work, but both artifacts and the way browsers handle JPEG color can work against a perfect blend.
Use the matching technique for most images with drop shadows, or other edges that are not clearly defined. Transparency can make a graphic look blocky depending on the image and how it lays over the background.
Keep in mind that transparency is not the best solution to every problem, and that Photoshop and digital imaging offer different techniques for different effects and results. Use transparency when it helps you complete a project rather than when it is interesting or fun to do. It isn't necessary all the time, and there are often alternatives to transparency use (e.g. layers and tables) if it creates more problems than it solves.
Understanding web color use, the properties of JPEG and GIF files and image implementation via HTML is essential to using transparency effectively on the web. There are some tricks to working with the edges of transparent, floating objects. Become familiar with the defringe and matting commands/features.
Working with selection is also essential to getting the results you want. Expertise in creating and manipulating selections requires understanding many of Photoshop's tools, and working at aquiring that understanding for more tools will allow you to more easily and accurately create the selection shapes you want for your images.
Be aware of the additional image space around your transparent image. Though you might only see a small area you intend in the final application, the file still retains all the pixels in the clipped or transparent area Ð and those take up memory and create file size. For better efficiency with the use of the images, crop the visible portion of the image as tightly as possible to the area you will actually be using. This will save load time in a browser, by keeping file size small.