In my own experience from developing myself in Flexicolor, Portra 400 can work well at 1600, and pushing beats underexposing with no push imho.

I almost never underexpose my Portra, because I mainly use it for portraits. It excels @ box speed and @ 320 in this regard with normal processing.

Portra 160 did handle underexposure well (1-2 stops) without pushing as well... it was from when I was excited to shoot it as it had just become available and the light had faded too much for normal exposure.

Invest in a tripod if it's a problem.

Underxposure and pushing is useable, but make it your last resort after exhausting every other option.