You need to use common sense and prevent any system that is vulnerable from freezing, so it all depends on how well your installation and insulation has been.
On a good system, all pipework in attics will be well insulated, and pipework under ground will be deep enough to be below the frost zone. If your attic is well insulated, but the pipework and tank aren't, then you have a potential attic problem, and should consider (in the short term) leaving the trapdoor open and in the long term, get that insulation onto your pipework.
But leaving on the heating 24/7 in all situations is wasteful. If you have a boiler in an outside shed, and you're worried that the pipes between it and the house might freeze, you could run it for a couple of minutes each hour.
There isn't a one size fits all solution.