NO. Leave a few joints unsoldered to allow for contraction and expansion that occurs w/ changes in seasons, humidity, temperature, etc. This of course assumes each rail end is clean, bare metal, and each joining rail is making equal contact w/ the rail joiner. Additionally, this assumes you have adequate quantities of feeders dropped along the rails. Also, it is important that your bus wires are of adequate wire gauge size. But the answer is NO.
Leaving unsoldered rail joins for expansion or contraction is a good plan. The question of how far between expansion gaps is likely to draw answers ranging from under 10 feet to over 15. Most modelers who solder joins do so to ensure electrical continuity. On my layout, I solder all hidden tracks, and most tracks through curves. I also have 24 gauge wire feeders soldered about every 6 to 8 feet, so it is likely that almost every piece of track will have at least one electrical feed that is soldered directly to it or it is soldered to a piece of track that has a soldered feeder. In that way, I don't have to depend on the joiner alone to ensure electrical continuity.
i don't solder any joints but i have power feeders on almost every joint. some high-flexible wire soldered to the railjoiner. almost invisible and you never risk to melt the ties of the track.
Leaving occasional unsoldered expansion gaps is a good idea. If you make your layout in a cold month, you may also want to leave actual gaps when you lay the track to give it somewhere to expand to in hot months. (and vice-versa for tracks laid in hot months; no gaps required initially) Also, put the unsoldered joins on the straight sections (and not within about a foot of a curve either). Curves are more prone to kinking if unsoldered.. .. Or so I've been told.
