Here's the write up from Mandela seeds in regards to soil grows and flushing:
Can I flush on soil?
You should avoid watering more than normal and having a lot of run-off in soil grows. Heavy watering or flushing/rinsing the soil can cause bud mold (run-off also washes out nutrients). Instead, give the plants their daily requirements and water at the start of the light cycle. Even if they let the leaves hang a bit now and then in the final 10+ days it's better than over-watering at this stage. In principle you should never have to flush in soil cultivation. It leads to all sorts of complications like waterlogged soil, nutrient run-off, fungus, etc. If you already know you have made a mistake like over-watering, always wait for plant health to improve before taking steps further with feeding, etc. You can't "cure" anything with feeding unless there is a real nutrient deficiency at hand. You won't get a nutrient deficiency in soil if plants are in quality soil, have an adequate container size, and the pH of your water is OK. If the pH of your water is not within range you need to correct it... feeding would only stress the plants even more because they can't take up the nutrients in a substrate watered with a wrong pH. Flushing your soil prior to harvest does NOT improve aroma and taste of the buds. It DOES increase chances of getting bud mold within 48 hours and can easily lead to other fungal infections which threaten your crop. The smart way to increase aroma is to buy quality soil and mix it with beneficial aroma enhancers such as worm manure and compost. Rinsing substrates should only be applied to hydroponic cultivations. The idea is to water with a very low EC, special ripening products, or only pure water a few days or a week prior to harvest - again, there should be no need to flush large amounts of water through the system. In coco coir this can also lead to a soggy substrate and increase the risk of bud mold. - See more at:
http://mandalaseeds.com/FAQ#IN14