Not Shari, but here's my reasoning --
I want the option to use enough fragrance to get a strong scent in my soap. If I use most FOs at less than about 6%, the scent isn't usually strong enough. Since I think the IFRA guidelines are based on sound reasoning, I respect those guidelines and don't buy scents with an IFRA % of less than about 6%.
Paradisi and Carolyn are correct that IFRA guidelines are based on total product weight, not fat weight. IFRA guidelines apply to many different products that may or may not have any fats in them, so it makes no sense to use a soap maker's outlook when setting the guidelines.
One reason why many soapers base the amount of additives (including fragrance) on the fat weight is that the fat weight doesn't change over time. The total weight of a soap bar does change due to water evaporation during cure.
If you choose to base your fragrance calculation on the total weight, you really need to estimate the amount of water loss when calculating the weight of scent to use. You want the scent % to be correct at the time the consumer receives the soap, not at the time the soap is made.
If you only use scents with a high IFRA percentage, that builds in a safety margin. If you base the amount of scent on the weight of fats, that also builds in a safety margin.