I think the reason late winter/early spring is recommended is because this is when they start showing signs of life and so enable you to identify which buds you should cut back to.
With Group 3, cut them back to the lowest pair of growing buds (or to the ground if no buds below about a foot) if you want them to produce flowers as low as possible - most Group 3 types will easily climb to 5 to 10 feet before the flowers appear. I try to train the new shoots into horizontal growth to keep flowers at eye level or below. Be careful if you do that as the new stems snap off at a leaf joint very, very easily!
With Group 2, you can see which stems are alive in late winter/early spring and so cut out any dead sections, starting from the top and working back. I always remove a few of the older live stems too (picking those which have long bare sections) and cut those right back to ground level each year to encourage new shoots from below the soil (ie from the "root crown", which will be larger the deeper you originally planted them.) Occasionally I cut an entire plant down to below soil level if it has become too leggy. Last time I did that to one of my "The President" varieties, which had 3 main stems, I was rewarded with 11 new stems! Being a Group 2 meant that it was late flowering that year and only produced the one set of flowers, but the next year it was spectacular, with over a hundred blooms over a 6x6ft area!