Although you may be showing 92gb on your C;drive Acronis will only backup the system and files it needs, there are other things that are not needed to restore your drive.
In my opinion you shoud always do a full backup, some may disagree but if only one of an incremental backup is corrupted you have lost the lot.
Take my advic and use afull backup each time. It really does not take too much time and then you are sure of recovery.
I always keep at least 3 backups Son, Father, and Grandfather.
That way I know I am safe.