  Accurate and precise parameter estimation for compact binaries is essential
for gravitational wave (GW) science. Future decihertz GW detectors, like
B-DECIGO and MAGIS, are anticipated to realize exquisite precision in the
parameter estimation of stellar-mass compact binaries. Most of the compact
binaries are expected to have very small orbital eccentricities, but the small
eccentricities can have non-negligible effects on the parameter estimation due
to the high precision measurement in the detectors. Here we discuss the
accuracy requirements of GW waveform models for the parameter estimation of
stellar-mass eccentric binaries with B-DECIGO and MAGIS. We find that if the
quasi-circular GW waveform model is used, even very small eccentricity ($\sim
10^{-4}$) existing at GW frequency $0.1\, \text{Hz}$ induces systematic errors
that exceed statistical errors. We also show that eccentricity measurement in
the decihertz detectors is precise enough to detect the small eccentricity
($\sim 10^{-4}$). Ignoring the high-order Post-Newtonian (PN) corrections of
eccentricity effects does not make significant systematic errors. As a result,
including the 2 PN corrections of the effects will be enough for the accurate
eccentricity measurement.
