I know keeping a constant temperature for fermentation is important, but what about after primary fermentation has completed and is racked to the secondary? It probably wouldn't be good if the temp went into the upper 70's, but how important is a constant temp. at this stage?
not in your secondary. You can also check the yeast manufacturers website and see exactly what the temp range is for each different yeast. Some will do great in lower temps. As a general rule of thumb, keeping it at or near the lower side of the temperature range will result in a cleaner beer with less chance of off flavors.