Is it good practice to substitute a resistor by parallel and series-ing them?
The pros about using a single resistor over either parallel or series:


Less cost
Less wiring
Less space needed
Less error prone (one component less to fail)


The pros about using a combination of resistors:


You can select resistors from the ones you have if you have a limited number of values
When putting them parallel, you increase the power (Watts), see remark of greggo
(You reduce the average tolerance, however since you have to design for worst case situations, this does not help).
Reducing need of (re)loading reels for high production volume, see remark of Tom Carpenter
Reducing risk when one component breaks, and in some cases (personal injuries) even mandatory such as in e.g. ESD gear or EX certified gear, see remarks of SteveSh and rackandboneman.


However, in any professional (and serious hobby project), I would use only one resistor when possible, taking the closes/best resistor value and power into account.