JAE...since a lot of my practice is litigation support, failure investigation, and expert testimony, I use the same rate for all, whether it is investigation, research, deposition testimony, or trial testimony. I also charge the same rate for travel time.
The only distinction I make is between litigation and non-litigation projects. For litigation, my current rate is $200 per hour unless it is an existing client with whom I already have projects going or an existing relationship. For that condition, usually $175 per hour, though I still have a few hanging around for less. There is a chance I will raise my rate this year, though I haven't decide yet.
My commercial consulting rate varies between $150 and $175, mostly for specialized stuff. For public agencies, we usually are stuck with an audited rate based on salary X multiplier.
Arguments can be made for using higher rates for testimony than other activities on a project. These include...more personal pressure, being a slave to court schedules, etc. The opposite argument can be made that if you charge more for certain aspects, it is just a "gravy" add-on for the "high profile" work, which can be used to discredit you on the stand. My logic behind this all this is that my time is worth essentially the same all the time. That way I can honestly testify that I am not just tacking on charges because of the litigation (insurance companies always think you're out to screw them on fees anyway, so they'll use these things as arguments not to pay the whole bill).
The next argument that comes up is whether you should only do work for defense. I am a firm believer that my answers must be the same without regard to who pays the ticket. Sometimes my answers match the case of the defense, sometimes not. I do mostly defense work, but to maintain objectivity I will occasionally do plaintiff work if their premise fits with my assessment. I have never "shopped" the other side in a case and do not plan to do so. I have seen that happen, however.