I suppose 28 years of experience helps! We have a map book now, where we've highlighted a lot of the easy N roads - and marked those which are problematic because they go through too many towns or villages. We can now do almost all routes toll-free. The worst bit of all is the Calais to St Quentin stretch in northern France, so recently we've taken to going the other way - along towards Boulogne, then Abbeville, Rouen, Chartres, and via the A71 down towards the A75 and Clermont Ferrand (the free autoroute to the south).
If we're heading for the Ardeche region we have another 'favourite' route which takes us down to St Quentin (from Dunkerque you can get there with few tolls), then a real cross-country road from St Quentin via Soissons, and Sezanne, down to Troyes alongside the Seine (a lovely route), and our overnight stop at Lac de Panthier.
We haven't found the N roads to be at all dangerous - just some to be faster than others because most of the towns and villages have now (sensibly, like the French do) been by-passed. Mind you, I suppose it helps in that we're travelling in low season (May/June, or August/September) rather than in July when the whole of France goes on holiday.
One year (years ago when my sons were small) our lone Citroen featured on a French news programme about 'Le Retour' (the date towards the end of August when all the French go home) - and mentioned that 'only Les Anglais' were heading south! We were the only car heading south for miles - and yet traffic in the opposite direction was nose to tail. We saw the programme on the TV in the hotel we were staying in at the time. The strange thing was that on the way back (same hotel) the news programme featured the International Juggling Competition which was taking place outside the Town Hall in Leeds - and there, in pride of place watching, was Mr Val A's elderly mother, who they interviewed! Two lots of fame in one holiday!