i know for a fact that sb's do not hedge the majority of business. they have no need to. they know the following fact:
80% of customers will lose their margin deposit in 6 months through the use of stops or simply poor judgement. the revolving door of members on t2w is evidence of this. the gambling urge of the public in their belief of quick money while paying large spreads ensure this.
the other 20% will either damage their account so badly and just leave the remainder with the SB, or refund only to lose again. 5% may make money in which case they may hedge if the exposure is large enough. generally however there is no need to do this because the order book will generally balance itself to a large degree. if it doesnt, they will adjust the quotes to make the side in need of balance more attractive.
the sb company will also pick up & keep interest from aggregate customer deposits and also trade a large fraction of the deposits in their own book - on the understanding that it is extremely unlikely that all customers will want to close their accounts tomorrow - just like banks and insurance companies will invest customers deposits/premiums. free money!
i have this info from an exec of one of the larger sb firms a few years back at an industry event/party.