Always verify signatures
For greater system security, it is strongly recommended to avoid installing unsigned software. Always make sure that signing keys and signatures are correct and/or use mechanisms that heavily simplify and automate this process, like apt-get upgrades.
As a reminder, digital signatures are not a magic bullet. While they increase the certainty that no backdoor was introduced by a third party during transit, this does not mean the software is absolutely "backdoor-free". Learn more about this process and what digital signatures prove.