Do I really need to do this EVERY week?
By doing weekly water sampling a baseline can be established for your greenhouse. The baseline of bacteria, mold and yeast in your greenhouse will be different from that of another greenhouse. At this baseline your plants will be healthy and there would be no concern.
The concern comes in when the numbers jump from what your baseline is. At this point no obvious signs of disease will be present in your plants but could become present within 2-3 weeks of the water sample being taken.
By doing weekly water sampling you would be able to take preventative measures to ensure your plants stay healthy.
Another benefit of weekly water testing is ensuring that the machinery you have in place to clean your water is working. The lights may be on but is it working? Weekly water sampling will provide a peace of mind that your machinery is effectively cleaning your water.