I'm guessing most experts will agree that the light source used when grading coins can influence a coin's appearance. At least for the majority of collectors, a coin viewed in bright sunlight will not look the same as when examined under florescent lights. The florescent view will be at least somewhat different from what the coin looks like under a halogen light and light provided by an incandescent bulb will give yet a different appearance. The situation is further complicated when different types of light are combined in the viewing area. An extreme example might be examining coins in a florescent illuminated office while sitting next to a window and using a desk lamp.
Things become even more complicated when we consider that how each of us responds to various forms and intensity of light can be quite different. After training hundreds of people to grade coins, I've found that when given the choice the amount and type of light that makes a person comfortable when grading can be quite different. Even the placement of the light source will vary.
If you attend coin shows you will notice that even in a well lit room suitable for reading, many of the dealers have installed one or more powerful incandescent lights at their bourse table. The reason for doing this is to supply the dealer with a consistent area in which to view coins and to provide a light source that overwhelms and negates the influence of light coming from other sources. While I consider such an environment to be less than ideal for grading coins, many dealers adapt to using the close and powerful lights and don't feel comfortable grading coins under other conditions.
I have found it possible to properly and consistently grade coins under a variety of circumstances. For this reason, I can't say there is an ideal environment for grading coins. After many years of grading coins for a living, I have discovered a situation that works best for me.
The area in my office that I use as my grading station is a windowless corner. The tubes in the overhead florescent fixture have been removed. The only light I use is provided by a “soft white” 60-watt bulb suspended 12 inches above my desk. The top of my grading desk is covered with dull medium green cloth that absorbs light rather than reflecting it. I'm able to get by with the fairly low wattage bulb because its light is not being interfered with by other sources. For me, such a light eliminates the eyestrain I've previously encountered when working under powerful lamps or in a mixed light setting.
Because I may find it necessary to grade hundreds of coins in a day, I've been motivated to do lots of experimenting. The environment I've mentioned works well for me and I'm reasonably pleased with the consistency of my grading. You may find a very different situation more to your liking. In any case, I encourage you to experiment a bit to discover which lighting situation might be best for you. If you avoid making any prior assumptions you may be amazed at the results.