The short answer to this is “NO you do not”, but as always I’m going up go into more detail so you fully understand why. 

First of all, let’s look at the pros of mixing up your workouts:

  • Keeps things exciting 
  • Can optimise muscular damage (a mechanisms of muscle development).

Now let’s look at the cons:

  • It’s almost impossible to track progressive overload (progression with the weights/strength)
  • You generally end up doing the same shit each week, even if the days are different, the weeks and months end up being about the same.

Let’s take this to the extreme, on the one hand we have boring Bob who does the exact same session each time he trains, it’s a full body split with a 500m row at the end. On the other hand, we have Jimmy Spandex who does a different session every time he trains. 

Now as boring as Bob’s session is, he’s more likely to improve as he can accurately track his progress. As much as Jimmy is a fun guy, it will be much more challenging for him to accurately track his progress and he would likely be using the same weights each week.

Now, results are not everything, nor am I suggesting you do the same boring shit each time you train. So here is the solution - Design a plan that is based around your goals, have a different session for each day of the week, but continue that plan for 4-6 weeks, this allows you to accurately track progression and allows you to compete with yourself. Essentially, if you know how you performed last Monday, how much you squatted, how fast you ran, then you’re likely going to want to beat those times. Once you have done 4-6 weeks, change up the plan, but maybe increase volume, this is the basic fundamentals of programming.

In summary doing a random session each time you train means you’re not training, you’re working out, working out gets you results, but training gets you to your desired goal.