If the assessee is subject to wealth tax, he may have to pay 1% if the cash balance exceeds Rs.50,000/-.Otherwise I don' see any problem. Only thing one should be able to explain the sources of such cash balance.
No specific rules for maintaining the cash balance. It is depend up on the size of business and needed for business day to day transaction. But as per the Income tax section 40(A) payment made above 20,000/- through the cheque only.