I think showing a bit of willingness to go above and beyond to help out a client can be good for you. I wouldn't go as far as to work a 10 hour day every day but the odd time is ok and normally a client will let you start late next day or WFH in return.
I once offered to work nightshift for 2 weeks as the permies refused to do it. I ended up doing it for 4 years. I only needed to work 5-6 hours per night which allowed me to take on other short term contracts during the day. I was investigated by HMRC but proved I was outside IR35 as I worked alone, didn't take direction and had more than one client at the same time.
On the other hand, I wouldn't go onsite without a contract in place as I wouldn't be covered by any of my insurance policies. I explain this to the client and they tend to chase up the agency to get the contract out and signed.
I once offered to work nightshift for 2 weeks as the permies refused to do it. I ended up doing it for 4 years. I only needed to work 5-6 hours per night which allowed me to take on other short term contracts during the day. I was investigated by HMRC but proved I was outside IR35 as I worked alone, didn't take direction and had more than one client at the same time.

On the other hand, I wouldn't go onsite without a contract in place as I wouldn't be covered by any of my insurance policies. I explain this to the client and they tend to chase up the agency to get the contract out and signed.
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