We do warn about mixing them when the server is a newer version that the Studio. That we do not support. However the Studio is designed to be backwards compatible except where explicitly noted. For example the SP6 Studio can't be used to debug XSJS in an SP5 system due to major changes to the way the debugger works.
I know of no such major incompatibilities between SP7 and SP6. SP7 Studio has several major advantages such as the removal of the requirement for REGI and the native repository implementation. This results in very nice performance improvements even when used with older server versions. Personally I've been running my SP7 version of the Studio and still working with the TechEd server images (Revision 60 and 61) without any problems.