AMI with or without reboot on AWS EC2

With a proper shutdown, it's stateless,in cold form and filesystems are sound. Basically nothing is in buffer or "moving around" while its creating the image. I believe the image creation also creates a bit of overhead as well. Rebooting is the most ideal situation an image will be in during creation. This doesn't mean you can't snapshot while it's up or that it's always bad.

Assuming the image takes a perfect snapshot of the disk at a moment in time you are still at risk of inconsistent data - for example a DB may have started to update a record and has written half to disk while the other half is still in RAM.

Most things will handle this fine, some system will through a fit - for example I used to work with ClearCase and you were nearly guaranteed to get VOB corruption if you didn't shutdown the server to take your backups.