When you say detail it, I assume you mean professionally detailed?
It really depends upon what your personal definition of detailing is (it can mean so many things), why you would do it yourself, why you would get someone else to it, what you intend to do at the end of the PCP, how fussy you are about your cars, whether you consider professional detailing to be part of the cost of running a car (or an unnecessary extravagance), etc.
Whether it’s on PCP or not, my hunch is that you’ll wash or detail it the same as you would if you had paid cash, HP, PCP or leased it, because it will be your car for that period.
When I leased a car I didn’t want to get attached, as I knew it would go back. I wanted to view it as a tool, something to get a job done, something I wouldn’t care too much about. For this reason I got a white car so I could have it washed at a cheap car wash with minimal risk of visible damage. I knew that keeping it super clear wouldn’t change the financial outcome at the end, but damaging it would cost me.
I like having a nice clean car, and I like it done right - it’s important to me. However I would rather spend my spare time doing something more even important to me. So if I’m going to get someone else to do it then I want them to do as good as job as me, or ideally better, and I know that’s not cheap. So I have a professional detailer who looks after my other cars, and I really thought the leased car would be odd one out.
Despite what I had thought, it never did go to a cheap car wash, my detailer looked after it. It didn’t have any kind of paint correction (it was white after all, which limits the benefit of correction), and it never had an expensive ceramic coating. But it did get a maintenance session every two weeks and a biggie every quarter. It made no difference to the value when it was stolen, but I felt happy driving it.