I think I didn't state the problem clearly... Here are the steps to reproduce:
1. In Finder, open a folder on the root Mac drive, containing an .xlsx document
2. Select the .xlsx document
3. Press Command-I to Get Info
4. Under "Open With", select "Other"
5. Navigate to the VM's copy of Excel (in my case, /Parallels/VMs/Slate/Windows Applications/Microsoft Office Excel)
6. Check "Always Open With"
7. Click "Add"
8. Click "Change All..."
9. Select "Continue"
10. ...time passes...
11. Get Info on the same document will now say it "Open With" Numbers, not Excel.
Even when Finder says it opens with Numbers, Parallels SmartSelect > Edit > Excel Documents will still say Microsoft Office Excel.
I'm not sure exactly what happens in step 10 to make the associations go away. It's not just a guest reboot or a Parallels quit-and-restart. It could be a host reboot, or an Apple software update...