Just my 2 cents: If you stick on some part until it's perfected you might bore yourself right out of trying to learn any more.

I'm not trying to pin down sight reading myself, so I can't give my opinion on that, but in general, my personal idea would be to make sure I wasn't bogging down on details to the point I forgot the pleasure of the journey.

As I have posted elsewhere, a quote from my Dad: 'Don't get so hung up on theory that you burn yourself out on music'.

...But don't leave it at imperfect, either. My thought is that you can progress and get some satisfaction, and still go back to regularly practice those hard bits. :)