How much to fill the fuel tank?

On my 5th gen there was a metal bar in the fuel tank and I'm sure the manual said to fill to this bar(while on side stand). Now we have a vtec and there is no bar, so can I fill it to the top of the tank? Or is something missing? What would happen if I filled my 5th to the brim? 

Owners book 6gen, says to fill to the bottom of the filler neck and there should be no fuel in the filler neck. I used to fill my 6gen well into the neck without any issues, by a few k's down the road this level would soon drop anyway.

Owned a 03 that would  fill up to the top, my 08 will not.

I've been known to squeeze every drop I can into my '06 when I'm on a long haul. 

The space above the fill line is called vapor space. It is there to allow for thermal expansion of fuel when the temperature of the fuel gets above the fuel's temperature when it was stored in your gas station's underground tanks. The vapor space is designed to test your patience, but with enough persistence you can fill the tank to the point where you cover the vent. Most of the time, nothing happens when you do this as you will immediately start consuming fuel upon leaving the gas station. But say you live really close to your gas station (e.g. on the same block) and parked your bike at home in the sun on a hot day right after filling: you might see some fuel leak out of the tank vent. Fuel on the ground adds risk, so manufacturers add vapor space to mitigate that risk. 

If you want to fill per Honda's design intent, fill to the bottom of the fuel inlet. I'm not familiar with the 6th gen, but it should have a conical opening where the rubber gasket seals against. Fill to the bottom of that when storing the bike. When immediately riding for some time after filling, you can fill her to the brim. 

5 hours ago, VfrV4red00Rua

Anyone got any tips for removing rust from the tank? 5th gen. and is it difficult to fit a new fuel filter?

also which are the best spark plugs for the 5th gen, standard or iridium?

Modern Fuel injected bikes are easier to clean out fuel tanks because the fuel pump in the tank gives you a large hole to work through.  You can find many way to derision a tank, but either way remove the fuel pump assembly before you try & if using the old school gravel method, make a temporary blank plate for the hole. You could get it grit or glass bead blasted if someone has a head that will fit in the pump hole. Regardless of how you do it remember to seal it immediately it’s cleaned or it will rust again. There are special tank sealants made for this.

have fun.