A lot depends on what you ride and how you ride it... Although as far as i am concerned you should be buying the best you can afford, tyres are the one part of a bike that really shouldn't be compromised on. My thoughts would be that if you use a bike for commuting and or do a lot of motorway miles, then buy the right type of bike and therefore have more options to safely use harder wearing tyres due to different stresses they are likely to encounter etc.
If your riding a supersport bike then it is quite likely at somepoint you are going to push the tyres hard... Really not worth the consequences of finding out they are not up to the job :
