I am sure Charc put this somewhere but i thought it may be useful anyway.
I had seen some questions on regarding high to build up research levels and found this ... http://ogame.wikia.com/wiki/Research
It has a chart that shows you certain levels of research top out at a a particular level and anything beyond that is just wasting resources.Basically we don't want to spend money where we get no benefit.
Example: When Ion Technology reaches level 5 , anything beyond that provides no additional benefit.
Hope this helps.