Are bypass diodes in low voltage solar panels with parallel cells really needed?
It probably won't damage them. 

Any light absorbed by a solar cell will either be converted to heat or electricity. If the electricity generated is not drawn off then it will be absorbed by the intrinsic diode in the cell and converted to heat.   

The worst case is with no load and one panel shaded on all cells but one. Because the shaded cells in that panel are producing a lower voltage, the non-shaded one gets a greater proportion of the voltage produced by the other panels, as well as having to absorb about double the current (the current it is producing plus the current pushed into that panel from the other panels). In a 6 cell panel that one cell will have to dissipate about 2.25 times more power from generated current than it would normally. Thus that cell will become a 'hot spot' in the panel. 

Sounds bad, but the cell already dissipates ~75% of the incident light anyway, so total dissipation only increases by ~25% which won't heat it up much more. In a large panel it might be a problem, but small panels generally have a higher surface area to volume ratio so they can dissipate heat better.

Without blocking diodes any shaded panels will pull the voltage down a bit, but under load the effect is small. Diodes also drop voltage, so the loaded voltage will probably be lower with them than without. Suitable Schottky diodes (eg. 1N5820, rated for 3A max) could drop 0.3~0.4V at full current, which might be unacceptable on a system expecting 3.3V.