If you don't have a licence, it is unethical and potentially illegal to use the post-nominal. It is misrepresentation.As long as you're not stamping any documents, or representing yourself as "licensed" (to win a contract, say), the NCEES notification should be sufficient evidence in the unlikely event that anyone questions the "PE" on your email signature. On LinkedIn, simply attach the NCEES verification link in the space provided. It takes no effort to add the verbiage "Pending state approval".
On a somewhat related note: see this NSPE position statement for use of the post-nominal on business cards when and engineer IS licensed in a given state, but travels outside.
https://www.nspe.org/sites/default/files/resources/pdfs/Ethics/EthicsResources/EthicsCaseSearch/2004/BER 04-11-Approved-May2005.pdf