Thanks for the '-Inf' workaround.
It has proven useful in another case

Sometimes '+inf' is better.

Here is a case where neither '-inf' or '+inf' work.
(in bothsides-inf.png; the upper stripe should be in dark red)

[x, y] = ndgrid(0:10:100, 0:10:100);
data = x + y;
data(data > 150) = NaN;
data(data < 50) = NaN;
data(isnan(data)) = -inf;
c = contourf(x, y, data);

using +inf is worse (resulting in bothsides+inf.png).

Confirmed.  This is almost certainly related to bug #37501.