In plasmid-based transient transfection of mesenchymal stem cells, do I have to select for transfected cells and verify GOI expression via a reporter?
No and Yes, respectively.
No, you don't need to select for and generally you can't unless you use something like FACS to sort and capture cells expressing your marker/GOI.
Transient transfections are generally not selected for, if they are it is to make a more stable line, and either requires integration of the marker site into the genome (most common) or in some cases use of a plasmid and cell system that supports propagation of the plasmid.
With transient transfection it is normally a good idea, at least for the first few times, if not every time, to confirm that the plasmid is expressing as you expect. You can do this by immunological methods, most commonly western blotting (WB) or immunocytochemistry (ICC). ICC will give you a percentage transfection, which can be useful in assessing how well you would expect an overall population to respond to a stimulus and how well you can see that effect - what if only 10% of your cells transfected compared to 50% or 90%, but it can't tell you how much protein is being expressed. ICC can also tell you how cytotoxic your transfection is - commonly used methods like the Lipofectamine family of reagents are often in the 20-50% cell-death range. WB on the other hand gives you how strong your expression is - how much protein is there, but it can't tell you how many cells were transfected or how much cell-death there is.
In theory at least FACS can do both number and strength, but it would require calibration of your antibody by both western and ICC.
You can also look at RNA expressed off the plasmid, but this may not bear a direct relationship to how much protein is produced.