This is a timely question as I have just had the same discussion with my parish councillors about the reserves they hold and I have insisted that they reduce their reserves in the next financial year to a level that I consider covers all short term liabilities. In previous years, we had a pavilion renovation that we had to save for but thanks to grant funding, we didn't need to spend all the allocated reserves. So next year, we are renovating a playground, installing solar panels and an outdoor gym.
It was a battle but really, how can you justify any precept increase if the parish council is holding public money and not spending it?
Reserves should reflect the liabilities that the council has - so it is right to have money aside to cover any contractual commitments, rent, elections, emergency building repairs etc. It is the unearmarked reserves that are the black hole.
Also bear in mind that when applying for grants, you have to state how much you have in reserves and that would certainly have an effect on your success rate.