In the process of changing the cassette on one of my road bikes I've encountered a difference of opinion between two shops.
One shop changed my old cassette (Shimano) for the new one (SRAM) with no comments. Just a routine bit of business.
The second shop, in another state, when asked to change my other road bike's cassette to the new ratios really gave me a lesson on why not to change to SRAM. The explanation was that SRAM and Shimano parts may work together but not as well as Shimano for all parts. Plus, SRAM in a Shimano drive train will increase wear and thus the parts will not last as long. Also, the reverse is true if the drive train was originally SRAM; don't put Shimano parts in it.
So, SRAM or Shimano stay in your brand. Don't mix or there will be increased wear and decreased function? Or, doesn't make enough difference to worry about?