What to do with internally unused pins on a non-programmable device?
Do not connect those pins to anything.  If there would be a need or advantage to tying the pins a certain way, the data sheet would tell you so.

"Reserved" pins can have several meanings, including manufacturer testing, factory programming, or reservation for a future device that will be pin-compatible with this device.  Whatever the purpose, it's not intended for end use.  It may be an input, an output, bidirectional, or a special voltage.  If the data sheet tells you to tie it high or low, it might be to disable factory functions, prevent an ambiguous state, or eliminate noise.

"No connection" means there is no actual circuit connected to the pin.  It may or may not actually be connected to the die (e.g. to dissipate heat).

If the data sheet doesn't tell you what to do with the pin, leave a solder pad to help mechanically anchor the pin, but it should be otherwise unconnected.