Should I use a falsetto voice?

It is recommended that you avoid falsetto in speaking voice as it uses more breath and ultimately fatigues the voices. We speak much more than we sing so a more robust speech quality is necessary. Falsetto also limits volume. 

The ideal is to use a higher chest voice - this involves thinner vocal cords which you can learn to sustain over time for longer and longer periods. Hence the 3 stages of the app which corresponds to the stages of voice skill acquisition. It’s also advisable at first to use a mildly to moderately raised pitch and increase pitch if necessary after a few weeks rather than jumping straight to a high - very high pitch as this might well lead to strain . Try a higher chest voice based on the model in the app but avoid too high a pitch.