Really depends on what you are doing with the gun and if you have other guns you want it to be consistent with and have set up the same way. If it is a recreation or gun games pistol then don't worry about it. When the dot fails, and it will eventually, due to breakage, battery, or user error, then no big issue. If it is a defensive gun then things start to get more complicated. If all you plan to do is protect you and yours, then generally speaking distances will be very close and the use of the dot or irons may not even occur. For most it will be straight up index shooting. I don't advocate that but it is just a reality. In that case it really does not matter. But for someone who may take the fight to an evil doer, courageous citizen or police, then engagement distances may be longer. The dot is very beneficial as distance increases but when the dot fails, and once again at some point it will, due to breakage, battery, or user error, no type of alternative sighting method is going to be as good as properly co-witnessed irons in the optics window. So for the courageous citizen or police, co-witnessed irons are not an option, they are necessity.
As to what type of irons, that is very much personal preference. In my early dot days I did not want anything that distracted me form the dot. I was afraid I would mistake a bright front sight with the dot. That was ignorance and inexperience on my part. I actual prefer irons that are clearly visible in the event of dot failure, which will occur eventually. The only concern is the use of tritium sights if you are using night vision with the dot. In that case tritium can be an issue. Not as much so with dual tube night vision but this does not apply to most.
As far as irons filling the window too much, this again is a personal preference thing. I like my irons lower to give a more open window. Same reason I prefer lower 1/3 on rifle optics. That being said, if you are keeping both eyes open and are truly target focused, as you should be to fully exploit the benefits of the dot, then absolute or lower 1/3 is not that much of an issue.
This is how I define dot failure, which will happen:
Dot breakage (external or internal), Battery failure (bad battery or was not change at appropriate intervals), user error (dot not set to the appropriate intensity for the environment so not visible, through poor manipulation dot has been turned down and is not visible, dot was not on)