You may be correct that the sinsues are not supposed to be sterile.  I
know I have read it somewhere during my endless research.   It seems
like I have also read somewhere that a few isolated is not significant,
but moderate to many is.  Anyway, the point I am trying to make is that
the coagulase negative staph may possibly act as a pathogen when it
overgrows.  That was the only thing I ever had isolated until recently.
I think I've read that some others here have also had that as the only
bacteria  isolated when they were having significant problems.  Here is
an abstract from medline concerning this:

Ear Nose Throat J. 2004 Dec;83(12):836-8. Related Articles, Links

Bacteriology in patients with chronic sinusitis who have been medically
and surgically treated.

Yildirim A, Oh C, Erdem H, Kunt T.

Cumhuriyet University, Medical Faculty, Otolaryngology Head and Neck
Department, TR-58140 Sivas, Turkey. altan11@hotmail.com

Chronic sinusitis is a disease that afflicts a significant percentage
of the population and causes considerable long-term morbidity. The
common use of multiple broad-spectrum oral antibiotics and endoscopic
sinus surgery to treat this condition may alter the pathogenes that
promote persistence of chronic sinusitis. Forty-eight culture-positive
patients with chronic sinusitis who had been medically treated for at
least 3 months and had undergone sinus surgery were bacteriologically
evaluated. Swab specimens of the middle meatus and sphenoethmoid recess
were aseptically obtained endoscopically and cultured for aerobes.
Coagulase-negative staphylococci were the most common isolates (45.8%),
followed by Streptococcus pneumoniae (16.7%), Enterobacteriaceae
(16.7%), Staphylococcus aureus (10.4%), and Pseudomonas aeruginosa
(10.4%). Coagulase-negative staphylococci were the most frequently
isolated organisms in our study, as in many other studies. Despite the
significant predominance of these organisms, they have always been
assumed to be contaminants, and their presence in culture has been
discounted. Coagulase-negative S aureus may be a pathogen in the
chronic sinusitis process, and sensitivities of this isolate should be
obtained for evaluation and possible treatment of the disease.

Another one from Rochester:

The microbiology of chronic rhinosinusitis: results of a community
surveillance study.

Chan J, Hadley J.

Division of Otolaryngology, University of Rochester School of Medicine
and Dentistry, Rochester, N.Y., USA.

In view of the rapidly changing patterns of antibiotic resistance,
community surveillance studies are providing important information to
help guide practitioners in making their choice of antibiotics. For
this community surveillance study, we performed a retrospective chart
review of nasal and sinus culture data obtained from 83 patients with
typical symptoms of chronic rhinosinusitis who visited a community
otolaryngologist in Rochester, New York. Pathogens were isolated in 59
of these patients (71%). The most common were coagulase-negative
staphylococci (31% of isolates). Among the other isolated pathogens
were Hemophilus influenzae (25%), Streptococcus pneumoniae (12%),
Moraxella catarrhalis (10%), Pseudomonas aeruginosa (7%),
alpha-hemolytic streptococci (5%), and Staphylococcus aureus (3%).
Approximately 39% of the coagulase-negative staphylococci isolates were
resistant to penicillin. Some 20% of the H influenzae isolates were
beta-lactamase-positive, and 14% of all isolates were resistant to
multiple antibiotics. Approximately 12% of the 83 patients cultured
positive for multiple organisms. Our findings provide important
surveillance information about the resistance patterns of pathogens in
our area. Although the prevalence of beta-lactamase-positive H
influenzae that we observed was consistent with those of other reports,
we found a lower prevalence of polymicrobial flora. Our findings
suggest that culture- and sensitivity-directed therapy should be
effective in the treatment of chronic rhinosinusitis.