Do shields usually have components soldered on their UNDER-side? I just took a look at the few that I have, and for them, the back-side of the PCB just has traces, no components.
I ask because I am trying to make my own variant of an Arduino board and want to ensure enough vertical clearance, so that components on top of my board won't interfere with components on the underside of any shield inserted into my board.
I think you'd have to look at each shield on a case by case basis.
I have components on some of my shields. You need to be careful not to put a component right above something below you, such as the USB connector on arduino, if you assume your shield will be directly above arduino, not something else. So I don't put anything big on the underside anymore except for my phi-2 shield with 20X4 display. I have a trimpot on the under side. That is the only place that makes sense and other shields are all smaller than this so no issue with component colliding into something below it.
If you do put parts on the underside, consider how you will solder the parts with top-side parts already in place.
What the Good Doctor is trying to tell you in simple words is don't paint yourself into a corner or better remember on which side of the limb you are relative to the tree when you begin sawing off the limb. Placing components, particularly SMT parts on the back side of a PCB is not a great idea... adding a tenth of an inch on both edges won't materially affect it's price and will allow more room to get all the parts on one plane of the PCB. The trick is to make the board large enough to fit all the parts on one plane (top or bottom) EASILY and then start shrinking the outline of the board until all parts fit and the PCB fits as well. This is more or less a situation where parts are shuffled around until both conditions are met. I started all my designs with 1206 parts and then I had the luxury of being able to specify smaller parts where required for room. It is extremely do-able and most often the method I used to make my PCB's. Thru hole parts are no real challenge with the possible condition where you don't have room for the holes and then use large pads. This is typically done for pots, switches and connectors and moving any of those off board will free large area's of PCB for other parts. Good PCB design is a complex interactive rule driven process and the tools that do it well are expensive... Altium is an example of a rule driven router. I used it from Ver 3 to Ver 7. Version three was a manual PCB tool and a good schematic tool which is the basis for a good design. Version 7 also cost $6.000 Per seat, W/O maintenance (Factory help) @ $1500 per year. The schematic tool is basically a fancy database, storing the parts data including footprint Wire sizes (trace width and length can also be specified for high speed busses (transmission line effects are calculated).
The little packages like Eagle... require a lot more experience of the designer as you are coming to find.
Bob