As suggested by user2200044
(in terminal)
cd ~/Library/Containers/com.apple.Preview/Data/Library/Preferences/
defaults write com.apple.Preview PVPDFSuppressSidebarOnOpening true
A similar approach without terminal worked well:
find the Preview preferences file by searching for com.apple.Preview
open com.apple.Preview with a basic text editor like textWrangler (free, I have no affiliation)
do a search in the document for the word sidebar
**For me this did not yield the PVPDFSuppressSidebarOnOpening term, in fact that term didn't appear to be in the document, however I did find:
<key>PVPDFLastSidebarWidth</key>
<real>216</real>
Replace the value 216 (or whatever number might be there) with the value 0
Save the com.apple.Preview document, restart Preview, and your thumbnail sidebar opening issues should be gone!