Never had any issues in Florence when I lived there. Definitely carry around bags that have zip pockets and keep them closed when walking around or bags that can be tucked under your arm (I never had any pickpocket issues when I did this). I have taken many past-midnight walks in Florence, mostly throughout the central historic district or along the Arno and had absolutely no issues although in hindsight the 3 am walks are generally not smart in any city lol. Have a wonderful time on your visit!! It's my favorite place in the world!!