Depends on what you are imaging Andrew. Barlow up the DSLR if you are going to shoot the moon or sun (properly filtered of course
) or even a wide field of Jupiter and its moons. But if you are going for DSOs unless it's really bright like the trap in M42 then usually we are trying to go the other way. If you add a barlow your f/ratio goes up by the factor of the barlow (ie F5 scope ^ f/10 for 2x barlow)
The attached photo is a couple of years old, back in the time of filum. EOS500D with 2xbarlow+2Xteleconverter on F/10 Celestron refractor.