In Boston it is customary to put $1,000. with the offer and either 5% or 10% (minus the $1,000.) at the Purchase & Sale (P&S) signing. In competitive situations 10% is common in the most desirable neighborhoods. Sometimes buyers offer more ($2,500. or $5,000.) with their initial offer to help make their offer stand out from what may be a crowd.
Some years back 10% was customary in downtown Boston. Many Sellers remember this and expect to get a 10% deposit since this is what they had to pay when they bought. In the suburbs 5% is customary.