You're Correct!

To correctly address the golf ball at set up you need to work on creating a really good posture. Stand up completely straight.

To check this you could place a club from the back of your head, down your back, to the base of your spine. If you are standing up straight, the back of your head, the middle of your shoulders and the base of your spine will all be in contact with the club. Take your stance up with your feet shoulder width apart.

Tilt forward from your hips, keeping the back of your head, the middle of your shoulders and the base of your spine, all in contact with the club. You need to tilt forward enough so that your arms can hang free of your body. This position will set you up in a great posture, but you will find that in keeping your legs straight you get a great deal of pressure in the back of your thighs. This is because as you tilt forward in this manner, you are stretching your hamstrings and in doing this, you will feel discomfort in the back of your legs.

The correct amount of flex to therefore have in your knees, is enough to relieve this discomfort and take the pressure off your hamstrings. So flex your knees, or soften them enough to make your set up position more comfortable and to ease the stretch in your hamstrings. This will allow you to maintain the tilt forward in your spine, from your hips, which is crucial to create if you are going to make an effective golf swing.

If you really bend your knees during set up, you will adopt a position that is very similar to sitting. As you bend your knees more, your spine will tilt forward less and become too vertical, rather than tilt forward. Make sure you tilt forward from your hips with your spine first and then just soften your knees to make this position more comfortable.

With no flex in your knees at address, you will find it very uncomfortable to tilt forward from your hips with a straight back. As you do this, your hamstrings will need to stretch, causing discomfort in the back of your legs. Feeling this discomfort, you are less likely to tilt forward enough to create a good posture to swing from. Ensure that you soften your knees and as such, bend them slightly to take the pressure from the back of your legs whilst tilting your spine forward.

As long as your clubs are well fitted to your size you should not need to excessively bend or straighten your legs to accommodate the club length. Instead, your knee flex should remain consistent as you change through all of the clubs in your bag and your posture or spine angle should increase or decrease depending on the length of your golf club.