There is no percentage. Anyone who gives you one without looking at the specific listing is guessing, and usually costing you the house or the money.
"Offer 5% under" assumes every home is listed at the same relationship to its value. They are not. Some homes are priced under market deliberately to generate competition. Some are priced at market by an agent who did the work. Some are priced 15% over because the seller insisted.
Offering 5% under on the first is an insult that loses you the house. Offering 5% under on the third is still overpaying by a wide margin. The number is meaningless without the context.
In a competitive situation the winning offer is frequently not the highest. Possession date flexibility, a cleaner condition structure, a larger deposit, or simply being straightforward to deal with can all carry weight — particularly with a seller who has already had one deal collapse.
Conversely, stripping every condition to win is how buyers end up owning problems they never inspected. There is a version of a competitive offer that is still sensible, and it is worth constructing deliberately.