I only just saw this post but just in case you still havnt decided. The answer is a very clear " use the solfege stickers" I read music in abc format, one of my girls does too. I am learning solfege with softmozart along with my kids and I can very clearly see the benefits already! I can see it will be quite easy to switch to abc format later ( or even at the same time for myself) but would be difficult to learn the pitch without the solfege. I had every intention of learning in abc...and quickly changed my mind

you get more from the program than just learning to play a bit of piano if you choose the solfege. Those piano key stickers are worth there weight in gold!