Unless the videos are of unimportant content, and/or storage space is *VITALLY* important, I wouldn't.
If you don't mind degradation in visual quality, and really really need the storage space, just use default settings, with an average bitrate of some amount less than the source file, based on the size savings you're looking for.
For simplicity, Handbrake's "Matroska" present of H.265 MKV of the appropriate resolution would be appropriate. You can save even more space by choosing a lower resolution (720p for 1080p videos, for example.)
A quick example, I just transcoded a 4K-source home video (iPhone 7, pre-iOS 11 H.264, 40 seconds,) using the "H.264 MKV 1080p30" and "H.265 MKV 1080p30" presets in Handbrake. Note that the H.265 (HVEC) encode took about three times as long as the H.265 encode. (~11fps vs ~3fps.)
The H.264 is 49.0 MB, the H.265 is 29.7 MB. The quality does look identical to my eyes; but that is with it freshly transcoded from a higher-resolution source to each target. I would bet that there would be some visual degradation transcoding same-resolution. Not exactly a massive size savings.
Edit: Just did a transcode of the H.264 output file from the above to an H.265. Only 22.4 MB (so slightly over 50% size savings) but I can notice some transcoding blockiness.
I'd say if you're transcoding from a higher-resolution source to a lower-resolution source, might as well go H.265 in the process. But if you're keeping the resolution the same, probably not worth it unless you really don't care about the visual quality.