Is it better to check before you change variable

When in doubt, test for yourself!

I would suggest making a loop to try employing in three forms. I can't guarantee how many iterations it should go through, but I'd start with something like 1,000,000 for most operations and work your way up from there.

 // C#
 
 float startTimeStamp;
 float endTimeStamp;
 float timeElapsed;
 
 float valueToChange;
 float targetValue;
 
 void Update()
 {
     if(Input.GetKeyDown(KeyCode.Q))
     {
         valueToChange = 0.27f;
         targetValue = 3.8235f;
         startTimeStamp = Time.realtimeSinceStartup;
         for(int i = 0; i < 1000000; i++) // increase the count from 1,000,000 as necessary. If there's a huge hiccup in performance, your test conditions are ideal!
         {
             // Test only one of these at a time
             // Version 1 -- boolean test
             if(valueToChange != targetValue)
                 valueToChange = targetValue;
             
             // Version 2 -- no boolean test
             valueToChange = targetValue;
         
             // Version 3 -- Empty loop
             // Run an empty version as a means of ensuring there's no meaningful overhead for doing so.
             // A smart compiling will remove the loop entirely, but there's no harm in covering all your bases.
         }
         endTimeStamp = Time.realtimeSinceStartup;
         timeElapsed = endTimeStamp - startTimeStamp;
     Debug.Log("Start Time: " + startTimeStamp);
     Debug.Log("End Time: " + endTimeStamp);
     Debug.Log("Time Elapsed: " + startTimeStamp);
     }
 }

Why "Time.realtimeSinceStartup" rather than "Time.time"? That's simple! Time.time has a cap of... hmm... okay, I forget, but it was something like 0.25 seconds. So if something takes longer than that to process, you can no longer track the time taken. That said, realtimeSinceStartup isn't affected by Time.timeScale, so it's not the universal best choice for all situations.

Edit: Ah, right. Forgot the obvious third test case.