Unused output pins should just be left disconnected.
For unused input or I/O pins you need to read the datasheet and/or make some discisions. As another answer points out the datasheet for your simcom device says you should leave them floating, so that is what I would do in the first instance.
As a general rule though, a CMOS input should not be just left floating. Doing so can significantly increase the current draw of the chip as the transistors in the input buffer start to conduct.
One option for IO pins is to configure the pin as an output. Then you can just leave it disconnected. Downside is if you make a mistake as to which pins you should set as outputs you may end up shorting an output driver.
Some input pins may have internal pull-ups and/or pull-downs which can be enabled, these will hold the pin in a known state with less risk of mistakes than setting them as an output. In the case of a module there may also be pull-ups/pull-downs on the module PCB.
If you aren't hugely constrained for cost or space then an external pull up/down is often the way to go. This can also double up as a conviniant place to solder on a bodge wire when you find out you needed the pin after all.
Tieing inputs hard to the power rail or ground is something I preffer to avoid where possible. It may lead to high currents if a pin is inadverantly set as an output, and often makes it harder to modify the board when you realise you need the input after-all.