The main reason for using flash, is so that you can use smaller f/stops, which increases DOF.
For example.
Lets take the Sigma 105 macro lens. With an F stop of F/11 and camera to subject distance of 24" the area in focus would be 1/2".
Without flash you would maybe use an aperture of f/4 this would give you just .17" in focus, leaving no margin for error.
A quick check on my macros photos shows apertures of f/11 upwards, with many taken at f/22
I use the Canon twinlight macro flash which is fantastic. but prior to that I used a sigma gun fitted with a softbox
My first first experiment was with two very cheap slave flashguns from Jessops, approx £30. The following photo shows the setup and worked equally well with the onboard flash, but was a little cumbersome in the "field" The second photos skows the result
http://www.photoshop.com/accounts/b8...1f6eb55ff1e90f
http://www.photoshop.com/accounts/b8...8222661ce3ba80
As for focal lengths of macro lens, I use the Canon 100mm, but quite often switch to the Sigma 180mm in difficult subjects/areas.
Harry