Hey Meg --perhaps I just missed it in your post, but what kind of orchids are you growing?
I do use a bloom booster type fertilizer on my orchids just before and into their respective blooming seasons, and it does seem to help with bigger and more flowers. I haven't noticed a miraculous difference, however. Really if the plant is healthy you'll get nice flowers etc with a regular fertilizer. I'm sure for people who are more professional growers--shows etc etc that the slight difference is noticeable but for the average hobby grower, it's just a nice little bonus.
Bottom line: I'd say that as a general rule, it probably isn't necessary for most orchids, particularly if you are using a good regular fertilizer. The bloom boosters primarily provide higher phosphate levels. If you look at your fertilizer bottle, somewhere it will have three numbers. Regular fertilizer is fairly balanced for example 7-7-7 -- that means 7% Nitrogen, 7% Phosphate and 7% Potash. There will be other elements too, but those are the big three. A bloom booster will be more like 3-12-6 -- lower nitrogen and potash with higher relative phosphate.