This, and other comments to the effect of purposely messing up your fingerprint to force PIN/password entry, are all technically effective. However, if you perform any of these actions after an encounter with law enforcement or the legal system has already started, it strikes me that your actions could be interpreted as destruction of evidence. IANAL, though.

Even using a PIN over a fingerprint is enough to piss off the FBI by making it more difficult to get in. If you're unlocking your iDevice with Touch ID only, you should consider your device insecure against even undetermined law enforcement.