By all means use 11. I do that every day here. There are very few issues you'll have to deal with here. Just be careful not to add a filemaker 11 only feature (it's a short list) and expect filemaker 10 users to be able to see and use them. On the plus side, filemaker 11 gives you a much nicer interface for developing and you'll be able to take advantage of the inspector, easier layout management and drilling down from a button or script trigger to the actual script for editing that you can't with filemaker 10.