I don't think there's necessarily anything wrong with tailoring your charts to appear a certain way or to project the kind of taste that you envision. If that's how you make use of last.fm, then go for it.
Although in my personal experience, being overly selective of what I scrobble, specifically trying to tinker with stats, and resetting to better reflect my tastes has always been more trouble than it's worth. To add to the futility of it all, my tastes tend to evolve and jump around a bit, and that's not always readily perceivable on the charts. Furthermore, the scrobbler app on my phone tends to scrobble even the songs that I skip, so that skews the stats a bit. Not to mention the fact that I usually scrobble a greater variety of music from my computer than I do on my phone.
So all in all, I think there are too many factors at play to worry too much about accuracy. I still tinker with a few of the display settings from time to time, but that's about it.