Is it safe to charge single 18650 cells without temperature sensing?
With a decent battery and charge-protect circuit it should be fine, since both the single-battery protection (using, for example, the popular DW01 chip) and charging IC (like TP4056) already limit the current through the 18650 cell; the current has nowhere else to flow, unlike the case with serial/parallel configurations.

If you look at the DW01 datasheet and it's typical application circuit (below), you can see that it actually monitors the current already and can completely disconnect the battery using M1 and M2 transistors in case of a failure. In parrallel and serial connected cells the IC does not monitor the currents between different cells and can't disconnect them from each other in case of a failure.

The temperature sensor, however, is still recommended for additional safety and redundancy, that is why some LiPo cells include a temperature sense output in their integrated protection circuit: for example, an external short circuit directly between the cell contacts situation could only be handled by a temperature sensor. This situation, however, could easily be avoided by designing a decent PCB and enclosure.