Building Worker first is far better than most of newbie strategies (which usually consist of not building Workers at all ;) )
But it's not that good either. If only because it takes time for newly founded city to build a worker, and all that time it does not grow. Better build workers in some mature cities, where it takes just a few turns to finish them. Especially in those which can not grow anyway at the moment (i.e. due to happiness cap). Obviously Worker First is the right game start, because you have the only city at that moment.
- At least 1 per city is not enough. Usually the right number is about 1.5 per city. You can adjust it according to circumstances if you really know what you are doing, but just blindly following this 1.5 per city rule is enough up to Emperor at least.
- Well, you build workers whenever you don't have enough of them (1.5 per city). Be aware that improvements are not carved in stone, quite often they need to be changed at later eras. Especially improvements around your newly
conquered liberated cities, previously built by AI.
- I wouldn't advice deleting workers unless you really really know what you are doing, are aware of the situation, are sure that no improvements need to be changed soon, and that maintenance cost is really too high to live with, and there are no other means to deal with it.
The goal is to never work unimproved tiles. Ideally no city should ever do that.
Hope that helps