So I quieten things down by piping to null:
aspnet_setreg -k:SOFTWARE\MyCompany\MyApp -u:"%username%" -p:"%password%" >:null
...and listen for ERRORLEVEL instead.
But recently I discovered that this fails with Access Denied if you don't have write permissions to the current directory (yes, you need write permissions to write to :null. Just don't ask). Not only that, but the whole statement fails, not just the piping bit.
So these days I pipe to %temp%\null instead.