Here's my list of things to do against biters I find almost universal. Prioritize them over everything else as you like:

1. From the very beginning rush grenades and heavy ammo, get some fish and go clear all nearby biter nests with grenades (before 0.2 evolution);
2. Get a couple of 4 turret stacks (full with red ammo) and put them on those sides of your base where you think you gonna get attacked from (cover turret stacks with pipes or walls). I'd advise to not mass produce turrets/red ammo, it's a waist of resources in the early game and they're not convenient to supply if you have a big base; 3 (additional). If you need to clear big nests after 0.2 evolution (and grenades are not enough) get a car with red ammo, grenades and a couple of red ammo research upgrades;
3. Get the flame turrets asap (no need to automate military science for it, you can craft 25 packs by hand) and set them up with a strong wall perimeter (with dragon teeth) around your facilities/outposts. This type of defense will last you to the end of the game;
4. Get the tank and explosive shells, now as your factory expands you can go and manually clear nearby nests;
5. Research robots, place roboports alongside your walls so robots could automatically repair any damage. Now your perimeter is self-repairing, this will last you to the end of the game;
6. (additional). Research artillery, build a train route that starts at the artillery shell factory and goes to every far point of your base (and to every outpost you have). You now automated clearing out nests in the general proximity of your base;

Should I build up defenses or should I pre-emptively strike?

Before getting flamethrowers you should pre-emptively strike every base you feel you'll be constantly "supplying" with pollution. After getting flamethrowers it's only defending with clearing up stuff only for expansion needs.