There is no wrong answer. You can code bills to expense accounts or the balance sheet. There are considerations for both options.

If you code to expense accounts, ScaleXP will know to journal the revenue to your default balance sheet account if you only use 1. If you use multiple balance sheet accounts for prepayments, you'll need to map prepayment accounts to expense accounts to ensure the cost is allocated to the right balance sheet account. On a monthly basis, you will go to the Identify Prepaid Bills page to choose appropriate prepayments then you will write your journals to push a portion of the costs to the balance sheet and then pull down from the balance sheet every month.

See here for more information when coding to expense accounts How to automate prepaid expenses: Setting up prepaid expenses in ScaleXP 

If you code to the balance sheet, and you need the costs to end in multiple expense accounts, you will be given the opportunity to choose the proper expense account for each supplier, bill or line item to end up in for the journal. You do this in Allocate Prepaid Bills.