The current situation (in WLK 1.4) for WHQL-signing is:
Win7 + WDF 1.9: No coinstaller restrictions. This means that you can use any WDF 1.9 coinstaller (beta, RC, RTM, intermediate builds, etc) or you can use no coinstaller. Since the coinstaller is not used to update a Win7 system, we wanted to allow driver submissions before Win7 RTMed, so this is a temporary policy. Now that the WDF 1.9 RTM coinstallers are out, it is suggested that you use those. We might update WLK 1.4 with a QFE to make the usage of the RTM coinstallers mandatory.
Win7 + WDF 1.0-1.7: You can use either an RTM coinstaller or no coinstaller
Windows 2000-Vista + WDF 1.0-1.9: You can use any RTM WDF coinstaller or no coinstaller (of course, you need to make sure that the needed version of WDF is already inbox if you don't submit a coinstaller)
After WLK 1.5 comes out (October 2009) the WHQL-signing situation will be:
For all operating systems and all versions of WDF (1.0 - 1.9) you can use either an RTM coinstaller or no coinstaller (if you don't submit a coinstaller, then the same restrictions as above will apply).