What files in a Maven project should be committed to git?

Personally I use Maven gitignore and Java gitignore for a Maven project. You might need to adjust it with the languages used in your Maven project.

https://github.com/github/gitignore/blob/master/Maven.gitignore

target/
pom.xml.tag
pom.xml.releaseBackup
pom.xml.versionsBackup
pom.xml.next
release.properties
dependency-reduced-pom.xml
buildNumber.properties
.mvn/timing.properties
# https://github.com/takari/maven-wrapper#usage-without-binary-jar
.mvn/wrapper/maven-wrapper.jar

https://github.com/github/gitignore/blob/master/Java.gitignore

# Compiled class file
*.class

# Log file
*.log

# BlueJ files
*.ctxt

# Mobile Tools for Java (J2ME)
.mtj.tmp/

# Package Files #
*.jar
*.war
*.nar
*.ear
*.zip
*.tar.gz
*.rar

# virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
hs_err_pid*

Is it good practice to perform mvn clean before committing, or do I add certain files to the .gitignore file?

Add rules to your .gitignore file first, which makes Git ignores the undesired files correctly. Understanding Maven standard directory layout will also help you better determine which are the undesired directories.

Check this:

https://www.gitignore.io/api/maven

In general you should ignore all targets and metadata. If you ignore targets, mvn clean is not required before pushing.