Generally, flight prices rise as you get closer to departure date. This is especially true with low-cost airlines. You will get the cheapest flight prices if you book well in advance. Rare execeptions are packaged holidays; so, for example, if you want to go from Stuttgart to Mallorca for a long weekend, you might do well with the travel sellers on the mezzanine level of STR airport.

However… Western Europe is fairly compact. I’d only use flight hops for distance longer than ~ 6 hours by train. Also consider busses such as Eurolines and Megabus for shorter distances if the price is really good.

So… long story short… I’d consider going ahead and staking out the longer flights. LIS to ROM, or DUB to ZAG, for example. Just the really long stretches with nothing inbetween that I plan to (re)visit. Once you stake those out with advance purchase airfares, then everything else inbetween can be as flexible as you want. Often, advance purchase airfares on Ryanair or Easyjet are so cheap, that even if you ditch your plans at the last-minute, you’re only out, say, $50-$60; whereas buying last-minute airfare would probably add $150-$200 premium on top of the advance-purchase price.