There are differing views on this question and I suppose it all comes down to how comfortable you feel and your personal cash flow situation, along with the quality of your properties. Ask yourself what could potentially go wrong and do you have the funds to fix it in an emergency? Many landlords allocated one months rent a year to spend on their properties but if your boiler and your roof has issues this may not be enough to cover it.
Maintenance is often very seasonal, boilers going in winter as they are used heavily. Be prepared for a few more maintenance jobs in the winter months in my opinions as people are in their properties more. Property is a business where cash flow is key to success. If you have sufficient funds in the pot then if a boiler does break it should not be a huge stress for you and your tenant. You have an obligation to your tenant to provide them with a habitable home so ensuring you can repair any issues is an important part of your responsibility.