Firenze > Venezia Train Reservations

If you are 100% of the day and time you wish to travel, go ahead and make a reservation . You will save $$ (€€) but advance tickets are often non-refundable so be sure you will use them. If you miss the train, you have to buy new tix.

Frecciargento (high speed) trains depart in the morning at:
8:30, 9:30, 10:15, 10:30, 11:30, 12:15, 12:30, etc. in the PM. So basically there is a train every hour or more often.
I checked the availability now for tomorrow morning (it's evening in Italy now) and all trains have available seats. However there are peak days where you show up at the station and seats (especially the cheaper 2nd class) are sold out for the very next train, and you might have to wait for the next train one hour later or even two trains, later. However first class tickets are very rarely sold out on any trains. So in case the 2nd class is sold out at 9:30, it's likely that you can still find the first class seat (at 50% more). A lot depends on when you travel. But if you go to the station or any travel agency in Florence a day or two before your departure date, you are guaranteed to find a seat. Also if you are absolutely sure of your travel plans, you could try to purchase online in advance (120 days prior) and try to get a supereconomy ticket at a fraction of the cost. However be aware that discounted tickets aren't refundable, so if you change your mind later you lose your money.

Now you know your options and have to decide how firm your goal is.