I have a holiday planned and don't know whether it is better to book the flights as soon as they are released or to wait and see if the price goes down. Any advice please?
there's no answer to this. Prices go up if there's a demand, down if there isn't enough; maybe higher midweek or weekend depending on destination... etc.
Personally, if there was a risk of flights selling out quickly and me having to walk, I'd do it early, but I can't guarantee it'd be cheaper.
Book as soon as you can. The airline wants to shift seats. You also get a full choice of options as to seating etc. The exception is with Club Class and the like; there you may get a better price close to departure, since Club makes BA a lot of profit and so there's an incentive for them to make offers to persuade passengers to fill it; this may be best achieved by you by upgrading near the time, rather than booking it as first choice. The airline might not want to advertise to Club bookers that they are discounting by the back door by upgrades.
Hotels are another matter, but you didn't ask about them.
Personally I'd book as soon as possible, especially if using one of the cheaper airlines. Prices do tend to go up rather than down, except at the last minute if they haven't sold them all.
I fly maybe every two or three weeks. Quite often, I just turn up at the airport and buy a ticket when I get there. You can get really good deals. Although obviously a bit disappointing if you have to fly, and there are no seats.
If it's Ryan Air or Easy Jet I'd book sooner rather than later, if it's someone like Lufthansa, sometimes their prices are so expensive to start with theyre underbooked and they occasionally drop theirs. Also if it's a cheapy airline, watch for their special offers, they can be very cheap with offer codes.
I tend to hang on and prices do go down at some point, some more than others. Budget airlines frequently have sales so you can get a bargain anytime if you are patient and are not bothered when you travel. That said, if you are tied to specfic dates and you are happy with the price, book it.