No, it doesn't cause problems, because FSX calls DX9 runtimes and you need to have DX9 installed, which is the DX version FSX uses. For DX10 many things changed from DX9 and in many cases you still need DX9 runtimes to run DX9 software, so there is just partly backwards compatibility with DX10 and 11 to DX9. DX11 versions are however able to run DX10 software, if you are using DX10 preview in FSX.It is believed that there's a 'new' version of DirectX coming in Windows 8.1, and it's this that is causing us grief.
Dave