I recently made my first Interrail trip for about 40 years .. it was great!

Now I am planning my second but I have not decided on a pass yet. But I have already made one seat reservation on the Eurostar from London to Paris.

Is that a normal / OK thing to do? .. first make seat reservations and then later decide which pass to buy to use these reservations with? (OK obviously you have to match 1st or 2nd class ticket/reservation)

It seems on the one hand like an odd way round to do it .. but it seems to work, and so why not? Is there anything I am missing as to why there would be any problem?