By shading all windows being hit by the sun you can save up to 25% of your cooling costs. With 80% screens a good portion of that savings may be obtained. Maximum savings will be achieved with 90% Suntex, but with a reduction in natural light.
The most common reason is because of the amount of light being blocked. If a a lot of light is preferred the 80% would be a better option. Common applications for Suntex 80 are windows where the sun exposure is minimal, or on any room where maximum natural light is preferred.
Suntex 90 does make a noticeable change in the lighting of your rooms. Blocking most of the sun's heat does require a reduction in the amount of light coming in. In the end it is a personal choice and only you can say how much light is needed in your home. If you are worried about savings and heat coming in, 90% would be your best choice. If you still want a lot of natural light but want to cut down on glare and UV rays, then 80 would be sufficient.
If you are trying see out at an extreme angle to the window then your view will be blocked more. Looking out the darker colors (brown and black) is easier then lighter colors but all are still easily viewable to the outside.
It's a personal choice, if you want a little more solar heat gain during the coldest winter months you can certainly remove the screens. Just take care in storing them that nothing pushes against the screen to stretch it out of shape.
Though they appear to simply twist off, the clips should be kept snug enough that high winds over time cannot work the screen loose. In most cases a turn with a Phillips screwdriver will loosen the clips enough to twist off. Remember to re-tighten when putting the screen back on.
If they are just dusty a light rinsing with a hose should easily take the dust off.
When dirt becomes "baked" on the screen they are a little harder to clean. A soft window scrubber and some detergent should do the trick. Be sure to rinse well to remove all the detergent. Also take care not to stretch the screen out of shape.
This is one of the selling points of some screen shops and in my opinion is simply a sales line. Solar screens do help shade the window but they are still a "partial" shade. As long as some sun is still hitting the glass the amount of heat radiating through the window frame is negligible. The real problem is solar energy easily penetrates the glass and then the resulting heat is trapped inside. All focus should be on shading the glass, and until all the direct rays are completely blocked the thermal conductivity of the frame is a non-issue. Windows will never insulate like a wall, but shading the glass from direct sun can significantly reduce your cooling costs.