It is implicit in a multiple-entry visa that subsequent trips will be more or less different from the first trip. For example, your second trip on this visa could be to another country, or another Czech city. Nobody is going to take issue with the fact that you've changed the place where you're staying.
A hotel booking is not required per se; it is just one way of showing that you will be conducting yourself legitimately while you are in the country. Specifically, one condition for entry of non-EU/EEA/EFTA citizens is that
they justify the purpose and conditions of the intended stay, and they have sufficient means of subsistence, both for the duration of the intended stay and for the return to their country of origin or transit to a third country into which they are certain to be admitted, or are in a position to acquire such means lawfully
A hotel booking is one element justifying the "conditions" of your intended stay. You can just as easily use an invitation from a friend for that purpose.
Caution: In United Kingdom visitor visa applications, at least, applicants visiting someone who rents their home need to show permission from the person's landlord. On the other hand, in some jurisdictions, this would be absurd because the landlord has no right to prevent a tenant from having guests.
I do not know how the Czech Republic views this issue. If your friend rents his or her house, you might want to investigate this question in more depth.
