There are a lot of views on what amount of space is required between copper foiled glass pieces. Some say the pieces should be tight, others that a consistent space is needed, and some who say that variable spaces are fine.
It is necessary to consider what holds a foiled panel together.
The foil is supplied with an impact adhesive which helps keep the foil attached to the glass before soldering. However, the heat of soldering deteriorates the adhesion of the glue. If you must take a foiled piece apart you will find that the adhesive is sticky rather than firm. Also, the adhesive will continue to degrade during the life of the object.
The solder bead is significant in creating the matrix required to hold the panel in one piece. The bead on each side holds the glass in place and resists deformation away from a single plane. This resistance is significantly reduced if there is not a fin of solder connecting the two beads. The beads and the fin of solder form an “I” beam which together resists movement of the glass.
To form that “I” beam there does need to be space between the foiled pieces. It does not need to be wide, but it does need to be enough to wiggle the pieces. This will allow the solder to flow from one bead to the one on the other side, forming a strong “I” beam.
In vertical panels, the glass is the strong element. The solder lines serve to hold the matrix together. Where people indicate the strong border will keep the whole panel from falling apart, they are correct in part. But, if there is not a sufficient “I” beam between each piece, the whole panel is subject bowing, either from wind pressure, vibration or mechanical pressure from handling. Therefore, you cannot rely on the border to make your panel strong and long lasting.
Some take the view that there will be enough unintentional spaces created between pieces to allow the fin form between beads intermittently. But the gaps in the “I” beam due to tight fiting pieces will make it much weaker than a continuous bridge between beads. The existence of gaps puts greater pressure on the solder that does bridge between beads.
An example was provided for me in a lamp brought in by client which spontaneously fell apart one evening. (Not made by me, I add). The upper band of glass remained attached to the vase cap, but separated from the rest of the shade. Fortunately, it fell straight down and only a little of the bottom edge was broken. Investigation showed there was very little solder between pieces, although there was a good bead on each side of the lamp. The lamp pieces separated, in different places, at the foil-glass interface and elsewhere at the foil to foil interface. This indicates there was little or no solder where the foil remained on the glass, as the adhesive is much weaker than even a thin fin of solder running between the inner and outer beads. This case is an example of the need for a fin of solder to be formed between the beads on either side to provide a strong, long lasting object.
There is sometimes a fear expressed that tight fitting of foiled pieces can lead to heat fractures when soldering due to expansion. Yes, when soldering pieces with a lot of variation in width, you do need to move reasonably quickly. Come back later to improve a bead if you need, to avoid overheating the glass. Even the thin copper foil can transmit heat along its length, which reduces direct heat transfer to the glass. Mostly, breaks occur from dwelling too long in one place with the soldering iron. It may be better to tin the foil all around the suspect piece just before running the bead. This will warm the glass around the edges in preparation for the greater heat of laying down the bead.
The main point is that the solder needs to connect the beads on either side of the glass to provide a stable, strong and long-lasting piece.