Back-up offers are typically made in competitive markets where several buyers may show interest in the same property.  Because there is always a chance that the first buyer may run into problems obtaining approval for his/her loan, if you are very interested in a particular home, it might be advisable to consider making a back-up offer. 

While your back-up offer is on the table, you should refrain from making an offer on any other home.  Therefore, your back-up offer should include a date and time when the offer expires.  Whether you consider a back-up offer will be driven by 1) your interest in the particular property and  2) the amount of time you are willing to wait before moving on to the next property.