Dummies are to provide similar operating enviroment for devices.
In the manufacture process, there are some variations of mask and echings.
Then the length or width u draw for a resistor or so may varies, even for the same one.
I guess it could be called of some kind of internal mismatch of devices.
30 years ago it was standard practice to put dummy resistors at the outside of resistor networks. These were there to produce the doping gas turbulence before it reached the real resistors.
Mainly dummies are placed to create same environment and to prtect the main resistor devices from the u nder and over etching processes in the manufacturing steps.The dummy should of the same width of the original devise and spacing also should be same as that of the internal strip spacing.
Example:
If u have res of W=3um and Number of strips =4 strips, spacing =0.3um.Then u need to place a single strip dummy resistor of 3 um width on the both sides .otherwise u can place a ring type dummy called etch guard(follow the same spacing and width). Dont forgot to connect it to ground in both the cases.
Better use first option as u specified.