So far, I've only been turning down those that are clearly outside of my purview -- sf, urban fantasy, horror, etc. Anything that had a strong female protagonist and felt like epic/high fantasy, I've asked for the first three chapters. I suppose if the writing in the synopsis were truly terrible, I might turn it down on those grounds too, but so far, that hasn't come up. If I do this for a long time, I may get more selective, but this is all so new to me that I'm inclined to cast a wide net for the moment, looking at as many manuscripts as seems reasonable.
I now have five sets-of-three-chapters in my inbox, sets that I've requested. I'm going to read those next -- I have no idea, at this point, what criteria I'll end up using to decide if I want to see entire manuscripts. This is a fascinating process, I have to say.