Reserves can be held at time 0. Ideally when a product is sold and funded at inception it should be self-supporting. This means that once reserves are established in the beginning of the policy, no further reserves should be needed in future to finance any negative cashflows. This is the reason why we zerorise negative cashflows to the beginning of the policy. Therefore, all negative cashflows must be adjusted such that only one phase of financing (i.e. setting up reserves) is required, which is usually at the beginning of the policy.
Furthermore, refer to negative reserves for an explanation as to why negative reserves may not necessarily mean a profit for the company and reserving basis which further explain why reserves at time 0 may be required for prudence.