In case of refusal, does the passport gets stamped?
I can say definitively that as of February 6, 2017 the USA no longer puts the Application Received visa stamps in passports to indicate refusals. Indeed consular officers are specifically instructed not to do so.
From the Department of State:
9 FAM 403.10-3(A)(8) (U) Indicating Nonimmigrant Visa Refusals in
(U) Do not place a stamp indicating “application received,” or any
other marking in an applicant’s passport in connection with a visa
application. With issuance and refusal data now available to all
posts through the Consular Consolidated Database (CCD), there is no
longer a need to alert interviewing officers to previous refusals by
making a marking in an applicant’s passport. In addition, CCD
information is now available at secondary in ports of entries (POEs)
and at other DHS offices. Officers at posts in Canada and Mexico
should ensure they follow the procedures in 9 FAM 403.10-3(A)(7) above
for refusing applicants who may have been eligible for automatic visa
revalidation at POEs.
The old procedure was:
PN1.2-13 Indicate Refusal in Passport
a. Posts must place a stamp in the applicant's passport to indicate
when a visa application is received and refused. The stamp is for
record keeping purposes, i.e., it will assist the post in locating
chronologically filed applications if the applicant reapplies for a
visa. If the applicant applies at a different post, it will
immediately alert the officer that the applicant has made a previous
application for a visa.
b. The stamp must contain the following text:
U.S. (Embassy/Consulate General/Consulate) (Name of Post)
Application Received on _____________(Date of Application).
The old stamp made it readily apparent to other consulates and everyone that you had been refused a US visa. Now as it stands, only the Five Eyes might know this. Does not mean it gives you the license to lie when applying for visas to other countries, however it opens up that avenue.
Should I be concerned with a visa refusal because I'm not complete sure I'll go through US now?
No visa application is every completely guaranteed and a visa refusal anywhere is generally a negative when applying for a visa to even a different country, so being concerned is normal and it is generally better to apply when you're convinced you will be using it.