When I used to fly that noble steed, I infact used 85kts but that's by the by. In my experince, the difference between IAS and TAS at a typical cruise of 2500ft is so small its hardly worth bothering with Of course, there is often a differnece and therefore if you want to be very acurate you should work it out on your flight computer before hand by looking at the temperature and winds aloft chart. Use your CRP to input altitude, temp and IAS to get TAS, then adjust it for wind as usual and get an accurate groundspeed.
In standard conditions when temperature is 15C at sea level, TAS is going to be approx 7% faster at 5000ft. so at 2500ft with IAS 85kt your TAS will be very roughly 88kts (i dont have my CRP here), not a lot in it is there
I guess to answer the question simply, i wither work out TAS correctly, or use my IAS cruise speed for 'bimbling'
I now fly an arrow at 130kts, and the TAS is often 10 or more kts different so worth considering, and if ever you fly in the states, where high crusie levels are common, its worth working out TAS accurately.