According to me stranded wire is good for carrying power(e.g. Vcc) but for carrying signal(e.g. 100MHz I2C traffic) I feel they create noise & since different strands might have some phase difference between them & since they are so close touching might put some noise in other strands. Also, the position of strands relative to each other can change over time with movement & handling. Solid wire seems to be better for carrying signal to me. I think I need some expert advice.
For short distances (e.g. < 0.1 m), stranded or not makes little difference for signals between ICs. I2C is not 100 MHz - max 2 MHz or so. In these cases, the greatest loss is from parasitic capacitance.
Strands touching or not makes no practical difference -- signals between ICs have enough noise margin to make any difference imperceptible.
you can expect a 3mm difference in strand length to distort edges at around 10ps.
On a single sided, or double sided circuit board you're un likely to be dealing with 10 picosecond edges. (thats like 50Ghz)
Use whichever style of wire is most convenient.