Hello Ankit,
My doubt is, why it will not be assigned?? I mean, are there some scenarios where it will not be assigned? If the assignment is happening inside some 'IF' condition then I'll probably suggest you to take it out making sure the field-symbol is assigned for sure.
Otherwise, wherever you are using the <FS> your'll have to check it if assigned or not which I think is tedious and unnecessary.
Thanks
Rajit 