This is a tip I learned to day from a great groomer. Since she told me I decided to share with everyone about it. Their are alot of debates if you should trim your shih-tzus hair in front of its nose or not. After trying it out my self I found growing it out and having it grow into the top knot to be very helpful and effective. Here is the reasons why.
1) when cutting the hair short it tends to stick in to your shih-tzus eyes. Always have to check to make sure it isn't poking your pup in the eye and irritating them.
2)sometimes makes it hard for them to see especially once it starts growing a bit
3) Having a topknot makes a shih-tzu look like well a shih-tzu and not just some dog with a long coat.
4) If you do decide this is the best route to go you should always keep your dogs hair near its nose trimmed at all times or else you will have the problem in number 1 and it can create a eye infection and more.
I think its best to ask your local vet or find a great shih-tzu forum and ask your self. People have different answers and may think cutting it short is wise and may make more valid points. But I have shih-tzus my self and tryed cutting it short and saw the results.
Here are helpful ways to keep your pups hair out of its eyes.
1) If it pets not a show dog or isn't going to be in a show I was told to use Vaseline. keeps your pets hair from going in its eyes and take a groomer band to push the hair in to the topknot.
2) If changing out your groomer bands please make sure to cut them out. Never pull them out as it can take alot of hair out.
3) keep a rubber band on it at all time and make sure to change out your bands 2 times a day.