Yes, you can absolutely use CarShake for free. The free tier is genuinely useful and not artificially limited to push you to paid. Many drivers use the free tier for years.
You never need to pay if: you valet park occasionally (less than once a week), you remember to delete old scans when you reach the 10-scan limit, and you don't need automated before/after comparison or PDF reports.
You might want to pay if: you're a frequent valet user (weekly or more), you manage a fleet of vehicles, you want cloud backup and cross-device access, or you want the strongest possible documentation for legal disputes (PDF reports are accepted by courts more readily than in-app screenshots).