The usual rule in planning climbs or descents when there is no appropriate page in the Flight Manual or Operating Data Manual is to use the climb (or descent) CAS and the FL, temperature and W/V for the 2/3 climb FL or the 1/2 descent FL. However, whilst this is generally good and will serve you well without being too much in error for an unknown aircraft, where there is a manual which gives specific climb and descent data for your particular aircraft, then you use the manual. (Or CAP 697 for the JAA ATPL FP exams). After all, it comes with the authority and stamp of approval of the manufacturer - it's not just some gash generally correct formula. The answers will be close whichever method you use, but not identical.
Keith is theoretically right when he says that TAS is up the slope but G/S is on the level, but practically the effect is negligible and the effect of the cosine of the climb angle should not be taken into account in Flight Planning. It might be appropriate for Performance.
Always use the data in the graphs. You don't actually need the TAS in the climb or descent and you should not calculate it when converting NAM into NGM in climbs or decsents for SEP and MEP. Extract the NAM from the graph then convert it into NGM by use of the following formula:
NGM = NAM + (W/C x climb time/60)
That is what the JAA are expecting to see as the answer.
Alles klar?