dicomread does not change the file in any way. Most medical viewers will use a subsampling. The viewer I use, uses an 8x8 bilinear interpolation (which increases the resolution on the screen to 4096x4096).
This can make the image clearer sometimes, but it is 'cheating'. You can use the altered image for visual analysis, but for numerical analyses you should use the original data in the DICOM file.
If you need a tool for changing the window level in an interactive way, you might be interested in my WindowLevel function. You can simply drag on the image to adjust the window level, which is especially useful for CT images, which have 12 bit (or in very rare cases 16 bit) data, while normal image functions assume 8 bits.