I think we should make a habit of stretching ourselves technically but we should not linger on it. The problem is that most people get overly obsessed over their troubles to play a very difficult passage without effort that it becomes an obsession which feeds inefficient study. They think that if they can solve this one technical problem in this one piece that they will improve. Yes it is true that they will improve if they add new techniques to their ability list but how much time should it take and should you dedicate your full attention to it? There is always that opportunity cost, 1 incredibly hard piece or few smaller pieces each which would build a stronger technical foundation. Difficult pieces can always be learnt on the side of your main study.
I remember when I was in primary school a library teacher told us, if you read one page of a book and you come across more than 5 words that you do not understand, then this book is too hard for you and you should try something easier. I apply a similar concept when I read music to determine if a piece is too hard for a student of mine.
In music there are usually sections which are easier and sections which are difficult, we have to know where to look for the difficult of a passages and this is different for each of us. We know we are in the clear when we notice passages that are easier and which we have experienced in past pieces, these can be disregarded, but when we get to difficult passages we have to measure how long it goes for and measure the % it takes up of the entire piece. We also have to measure how difficult it actually is for us to attain an effortless touch to produce the difficult passage, this means we have to actually sight read our music and experiment (to developing pianists sight reading and experimenting itself is a study they are developing and plays a decicive role in the speed music is learnt at since how can we tell if a passage is too hard to physically play if we firstly have difficulties using the information on the score!). We cannot be satisfied to be tense to produce the string of notes, that is not highlighting technical control over your playing.
Everyone goes about a different logical process to determine actually how difficult a passage is for them. For me I simply look at the hands of my students as they play (after they sight read a passage as accurately as they can) this tells me basically everything I need to know about the difficulties they are having. But you should also address the thought process going on in your mind, how are you visualising playing the patterns on the keys and in the fingering etc.
I have a young student who plays tough pieces with good technical control (he looks very comfrotable playing) but with very little musical value in this playing. The simply doesn't care about playing notes beautifully rather he just wants to be able to play all the notes without note errors. He is very stubborn and will not let me spend time perfecting a piece, once he can play the notes he wants to move on. I accept that some peoples musical maturity will need time to develop, when they actually start caring about playing music beautifully and stop obsessing about playing the notes.
Topic: When to back off? How? How much to push?