Spend it when you have it! Cash is the blood of your business and it accelerates development.
To a certain extent only, as too much cash isn't efficient. The same way a large company will burn money at a much higher rate than a smaller one, due to complex processes and reduced reactivity, a little cash will go a long way.
Pay attention to your financials and be wise with your daily decisions. Make sure you spend it only when you need it, it'll help you raise more money later if you can prove you're good at managing it.