PlayerPrefs save after quit?

I would say you do not really need to save in the Update. Since you are on iOS you can use:

http://docs.unity3d.com/ScriptReference/MonoBehaviour.OnApplicationPause.html

or:

http://docs.unity3d.com/ScriptReference/MonoBehaviour.OnApplicationQuit.html

but make sure you read the bottom line of the page about the iOS not quitting.

Then you are getting the value on Start and again you do not need to check in Update, create a method to update the score value and at the same time check if bigger than the HiScore:

 private var highScore:int;
 private var score:int;
 private var maxValue:int = 10000000;
 
 function Start()
 {
    highScore = PlayerPrefs.GetInt("highscore");
    highscoretext.text = highScore.ToString(); 
 }
 
 function AddScore(newScore:int)
 {
     if(newScore < 0)
     {
         Debug.LogError("Negative value?...Really?");
         return;
     }else if (newScore > maxScore)
     {
         Debug.LogError("That is a big score??!!");
         return;
     }
     score += newScore;
     if(score > highScore)
     {
         highScore = score; 
         highscoretext.text = highScore.ToString(); 
     }
 }
 
 function OnApplicationPause(pauseStatus: boolean) // Or Quit
 {
     PlayerPrefs.SetInt("Highscore", high_score);
     PlayerPrefs.Save();
 }

grab the save and set out of update and put it in your quit(exit) button

  function exitFromGame()
       {
        if(currentScore>high_score)
         PlayerPrefs.SetInt("Highscore", currentScore);
        PlayerPrefs.Save();
      }