The dimension of your bitmap does matter for downscaling bitmaps. Downscaling is the process of creating smaller and larger dimensions of a bitmap in place of the original art to improve scaling images. This process is done directly from the original to the desired size. It was precomputed in previous versions of Flash Player.
Both width and height values must be an even number. A perfect bitmap is one with dimensions that are a power of two, but it is enough if the dimensions are divisible by eight. The following numbers demonstrate downscaling a bitmap for the resolution of a Nexus One and a Samsung Galaxy Tab. The mip levels, scaling values reduced in half, work out perfectly:
800×480 > 400×240 > 200×120 > 100×60
1024×600 > 512×300 > 256×150 > 128×75
A bitmap at 1,000 by 200 would reduce to three mip levels, but a bitmap at 998 by 200 only reduces to one level.
The point to take away from this is that you should create assets that fit within these dimensions.