Update

Percy, I'll be honest.  When I had my surgery, both surgeons I consulted (at two different facilities) were insistent that I had no choice about the SNB.  If my diagnosis had been pure DCIS, they might have agreed to give me a pass (I was having a MX so normally an SNB is done) but with the microinvasion, there was no way that they would have allowed me to go without the SNB. 

Here are a few articles about this issue:

From 2011: Role of Axillary Staging in Women Diagnosed With Ductal Carcinoma In Situ With Microinvasion   "In summary, this is the second largest series of consecutive patients diagnosed with DCISM.  Our data suggest that despite the low likelihood of nodal extension, a
lack of identifiable clinical and pathologic features
associated with a positive sentinel lymph node
precludes the use of selective sentinel lymph node biopsy. Thus, until
further
data are generated and validated, a sentinel lymph
node biopsy should be performed on all patients with DCISM diagnosed
either
on core needle biopsy or after a definitive
resection."  

From 2005:  Sentinel Lymph Node Biopsy in Intraductal Carcinoma of the Breast with Microinvasion    "From March
1996 to December 2002, out of 4602 SLNBs performed for invasive
carcinoma of the breast, 41 patients with DCISM in the definitive
diagnosis were selected. Metastasis in the SLN were detected in 4 of 41
(9.7%) patients. Two of the 4 patients had only micrometastasis in the
SLN. In three of these patients, the SLN was the only positive node
after CAD. SLN biopsy should be considered as a standard procedure in
DCISM patients."   

From 2000: Sentinel lymph node biopsy: is it indicated in patients with high-risk ductal carcinoma-in-situ and ductal carcinoma-in-situ with microinvasion?    "RESULTS: ...Of 31 patients with DCISM, 3 (10%) had positive sentinel nodes. 2 of 3 were positive for micrometastases only....  CONCLUSIONS:This study documents a high incidence of lymph node micrometastases as
detected by sentinel node biopsy in patients with high-risk DCIS and
DCISM. Although the biological significance of breast cancer
micrometastases remains unclear at this time, these findings suggest
that sentinel node biopsy should be considered in patients with
high-risk DCIS and DCISM."

I also reviewed the NCCN Treatment Guidelines (physician's version 1.2014) to see if there was any mention made of possibly being able to pass on having an SNB when the diagnosis is DCIS-Mi.  DCIS-Mi is grouped in with other Stage I cancers, and no exceptions to the SNB requirement were noted.  Within the DCIS section, the following was noted: "Patients found to have invasive disease at total mastectomy or
re-excision should be managed as stage l or stage ll
disease, including lymph node staging." NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines)® Breast Cancer     Note that you need to register on the NCCN site (it's free) in order to be able to access the physician's guides.  Patient Guides are available without registering but they don't provide the same amount of detail.

Percy, 

DCIS can only be outside the duct if it was placed there accidently - by a surgical instrument, for example.  It's been known to happen.  

IDC cells are different biologically from DCIS cells, but this difference can be very difficult to detect. Here is an interesting presentation that talks to this issue: Problems in the Diagnosis of Ductal Carcinoma in Situ    See pages 33 to 55.  Additional immunostains for myoepithelial cells may be done to try to determine if the cells are DCIS or IDC but my understanding is that this doesn't always provide the answer. See the discussion of In Situ v. Microinvasion near the bottom of this web page:     Immunohistochemistry in Breast Pathology   

As for HER2 testing, yes, this can be done on DCIS.  So the fact that the microinvasion will be tested for HER status doesn't tell you that this truly is a microinvasion, vs. it being DCIS.  As an FYI, approx. 40% (some studies have suggested higher) of DCIS is HER2+, whereas only 20% - 25% of invasive cancer is HER2+.  No one knows why this is.  And while HER2+ DCIS has been evaluated in many studies, most of the studies show there to be no difference between HER2+ DCIS and HER2- DCIS.  A few studies do show HER2+ DCIS to be more aggressive, but there have also been one or two studies that have shown HER2+ DCIS to be less aggressive.  With IDC, it is well established that HER2+ IDC is more aggressive.  So the reason that DCIS often isn't tested for HER2 status is not because it can't be done, but is because there is no known significance to HER2 status for DCIS.  There also is no difference in treatment between HER2+ and HER2- DCIS.