Mmmmmm, its more the probes that come with the mobo, such as Asus Probe, that are really off by sometimes 10C. But MBM tends to be more accurate, MBM is only about 1 or 2C off, so it shouldn make that much of a diff.
Well evryones MB will read different so if you listen to someone else's reported temps that does not mean yours will be the same. Try Sisandra and see how close its temps are to MBM, if your using the onboard thermistor make sure its touching the CPU but not too much as to interfere with the CPU sittimg in the socket all the way.
My advice: download as many monitoring programs as you can find. Compare the temps and see which ones are on the same page. Generally I have found that MBM5 is accuracte and the most user friendly. Plus I like to be able to pull up the highest temp and lowest temp during a session.
I would pay attention to your BIOS/Via Temps first and formost, as you said...MBM is in line w/ Via...so you should be good there. Sandra tends to either show high or low when compared to the BIOS on all of my boxes so I usually don't use it as a correct measure.
I went through this dilemma and finally broke down and bought a digital thermal probe. They really are the only way to know for sure what your temps are. My MBM5 on my AMD 500 system is usually off by about +/- 5C that's a lot to me, so I got a temp probe. If you decide to break down and buy one I think crazy pc has them for the going rate of 20 bucks. There well worth the investment.