Already purchase clubs

Most people would get the fitting first to ensure they are purchasing clubs that perform the best with their swing. Certainly won't hurt to go through a fitting now, but if your clubs don't truly fit your swing you may need to spend more to get them correctly fitted to you or you can play with clubs that don't fit you correctly

Too late now. The time for a fitting is BEFORE you purchase.

If you get a chance to do a true Titleist fitting, take it. It's a great experience and you might end up just fine with the clubs you selected, but I think you will be surprised learning how much difference some of the shafts can produce for you, and it will influence future purchases.

Too late now. Do the fitting just before you buy new clubs.

Doing a fitting now you can make sure the length lie and grip size are correct but if you would have done one before you bought them you could have made sure you have the right shaft and head.