I'm curious about something -- with the serious, completist collector, when is the best time to buy the rarest games for a system you are trying to build a complete set of?

For example, you are collecting Saturn (JP). You have a pretty good collection of say 200 titles and intend to complete the set some day, then you run across a Taromaru for, say, $200. Very hard to find, don't know when you'll come across one again, but it's $200 you could spend on a bunch of other titles. Assuming you have the cash and don't mind spending it, do you get the hard to find game now, or wait until you are further along in your collection to worry about it?

Do you work from the common titles to the uncommon ones to complete a collection, or just buy something you want, when you see it and can afford it?