Hi @MissisE - Our T&Cs and Code of Conduct specify in detail what is and what isn’t permitted when arranging house sits, in respect of personal needs and expectations. Making a request for a contribution towards utilities is a financial request that is permissible as long as it is stipulated clearly in the listing so that sitters can decide whether or not to apply in advance.
My personal experience - My partner and I are currently doing a long term sit (1 year +) where we are paying the electric bill, because the provision of a property without pets, the freedom to use as a base, in return for this payment and the garden maintenance means it is a win win for both parties. We’ve only ever considered this on a long-term sit, never on a short term sit where we would expect the home to be kept warm for us and the pets 
But this is the individual sitter’s choice.
Hope that helps.