So, as is the answer you will get in any case:
Much like turning off the switch before installing a lightbulb, it is best practice to use one. You could trigger a static discharge and that could mean sad days.
However, if you ground yourself frequently and avoid working on surfaces like carpet, you will probably be OK.
Nobody with sense is going to tell you it's 100% if you don't use one. It's not likely - but it can happen, and that's up to you to decide. My two cents? I've built 4 PCs now and never once used one - all have gone off without a hitch. But again, and I have to stress this: it remains a possibility. We cannot guarantee you will be safe not using one, and if you decide to do without, you have nobody but yourself to blame.