Tally

Yes it is mandatory to Group Ledgers under seperate catogories. 

Otherwise, P& L and B/S will not reflect the right figures. 

Yes, It is required to created a ledger under different group..

Group is nothing but specifying the nature of ledger you are creating..

Suppose you are to creating 2 ledgers.. 'Rent A/c' and 'Advance Rent A/c'...

Now How would tally know that if you are debiting something to 'Rent A/c' then is to be carried to P&L and if you are debiting something to 'Advance Rent A/c' then it is to be carried to Asset Side of Balance Sheet.

You have to tell the Tally that 'Rent A/c' is an Indirect Expenses and 'Advance Rent A/c' is a Current Asset. This is done by choosing the correct group of a ledger.