Cardiac DTI is subject to long scan times and low SNR, which invariably leads to a trade-off between the number of averages and diffusion-encoding directions to acquire. However, the exact relationship between the diffusion tensor and these parameters is unclear. In this work we utilise DTI data from five high quality ex-vivo rat hearts, and vary the SNR between 2 and 97 and the number of directions between 7 and 61. Results show that the apparent diffusion coefficient is optimised for scan time when SNR is maximised and directions are minimised, whereas parametric angle measurements are time-optimised with more directions. At typical in-vivo settings, we estimate that fractional anisotropy is being overestimated by up to 20%, while the precision of sheetlet angles may be as poor as ±36 degrees.