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.

If you are purchasing parts for an amplifier design that you will be tweaking resistor sizes to optimize performance I strongly suggest that you invest in a resistor assortment kit. Such a kit will prove useful for you for many future projects as well. I still regularly use resistors from an 0805 SMT kit that I purchased over 16 years ago.

Kits such as these give you a lot of flexibility to stick with the single resistor philosophy or in selecting unique in between values either by doing components in series or parallel.

You can get kits such as this one with axial leaded resistors:

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Alternatively kits are available with SMT components of a specific physical size. Here is one kit with 0603 sized resistors:

^{Screen Shot Source}

What they should have said, was that in practice you work with what you got and it often isn't necessary to use networks just to get a non-standard value. For that, you really want to get used to the E12/24/48/96 series.

For example, if you calculated you need 18800 Ohm, a 18k7 would usually be good enough. Often enough 18k will do fine. Not even talking about tolerances on those values (although the problem of tolerances does add-up if you start combining resistors, make sure you use those that are accurate enough if your application needs it), often enough simple is better than accurate.

The intent behind their statement was fine, but a bit too absolute. Sometimes, you do need a very specific value. However, in that case it could be wiser to put a single E192 resistor in it instead.

For prototyping purposes, having the entire E48 or better available is quite useful. Don't let anyone stop you from getting a range of resistors.

If you don't have a list already, find your E-series lists here.