I'd say it depends on the year.
If many people miss their offer one year, they may be more lenient as they want to fill seats. However, if it is an already oversubscribed course and many people have firmed it and gotten the grades, they may be less lenient. Always bear in mind that unis will send out offers to way more people than they have seats, expecting some to miss their offers (or not firm/insure the uni).
It is best to work as hard as you can to achieve the grades rather than to worry about missing your offer. Though, there is always a possibility of clearing just in case.