A few of the determinations in the literature for my analysis specify silanizing the glassware, but most of them don't. When do I perform this extra step? One paper implies that it is done for very dilute samples, and some of these determinations are dealing with picomolar samples. My samples would be in millimolar range. Can I ignore silanizing?

Noname,
You don't say your analyte, with that fact and the volume of your sample and how much glass ware it comes into contact, you could estimate the retention.
A surer way is to prepare your sample at half you lowest expected concentration. Expose it to treated and untreated glassware, or if you are super lazy just shoot from your master solution and one of which you put into clean glassware and see if you can see a loss and if you do is the loss acceptable.
Jim

I've had some luck using plasticware instead of silanizing for my work. In my case, it works as well for the short term, but after 3-4 weeks I can see a 15% difference. It is probably worth the effort to check it out.

I just track the concentration change over a period of time--usually 1,2,4,8,16,30 days.

If you treat your silanized glass carefully, you don't have to re-silanize every run, either. I use mine for the summer --about 30 runs-- and re-silanize it in the winter.