Very interesting and hopefully continuing discussion.
I have been very reluctant to use flash for my subjects as my limited experience, and probably simplistic approach, yielded very flat looking images, which is not bad for items such as phalaenopis, miltonias, vandas and the like. However, I like the use of the lights on stands (four total) as, when I light frontally with two and move two to the sides, the texture and three dimensional quality of the flowers is emphasized in the final photographs. One exciting aspect also is to selectively backlight some of the orchids such as cattleyas and Chinese paphiopedilums as they have wonderful veining and, with the cattleyas, the front surface under these conditions takes on an almost irredescent (sp???) glow. One of my cohorts (Greg Allikas) who does the photography down in Florida uses flash very successfully and I believe that Charles Marsden Fitch up in New York also does the same. Google their names to see some of their work as well as go to the American Orchid Society website.
As far as the focus point, I am a practical, simple man with an engineering background and tend to go with what I have found works well, whether I understand the complicated theory and concepts behind them. As I tell people who are trying to learn this type of photography, it is better to err on focusing too forward on the subject as opposed to back on the subject. An orchid photo with the foreground features sharp is a lot more appealing than one where the foreground features are out of focus and the background ones are in. The nice thing about today's optics are that they are superb when working with the shallow depth of field that is present in this type of photography.
One other item too is isolation of a single bloom, tricky at times, I use masking tape or sometimes roll solder wire to gently move/hold the arrangement to my liking.
Also too, on the subjects of backgrounds, dark flowers command a light background and vice versa. Seamless white paper, when unlit and placed about two to three feet behind usually provides a nice pearlescent grey color that works well with dark or light flowers.
Hope this proves informative. Have fun.
P.S. I do not use any filters, I even remove the UV protector when I photograph.