Crosstalk between parallel traces...

[westfw]

How much should I worry about running pin signals (say, microcontroller IO ports) as closely spaced parallel tracks on a PCB?

Back in the low-density TH days, I would have tried to keep traces pretty far apart as much as possible, even interspersing "sensitive" signals (eg analog inputs) with GND signals to form a sort of "shielded" connection, but this is getting increasingly impractical as finer-pitch SMT becomes more popular, and the math in my head (two "wires" approx .2 [ Attachment Invalid Or Does Not Exist ] mm apart and and less than 5cm in length; not much capacitance between them!) says maybe I've been silly all along.
For example, consider these two layouts.  Does it matter?

Here's some of the math:  http://4ms.org/?p=441

[selcuk]

Yes, it is hard to place GND between each signal. But the coupling is not only capacitive. There is also a fringing effect of EM radiation. You should place the ground plane under the signals very close to the top layer in order to keep the radiation distance small.

[2N3055]

Speed is the key. Namely, how fast signal edges are and signaling rate.
Look up Prof. Bogatin. He has some nice material on the topic.
That is for digital.

For analog, it depends on analog levels. Analog you shield as needed.
For analog, there are no rules of thumb.

[thm_w]

As mentioned, does it matter depends on your design and what signals they are. If they are simple low frequency 3.3V button inputs or LED outputs from a MCU, it does not matter at all. Is this a hobby design where we don't care too much about EMI?
Need context.

Also looking at that PCB layout my guess is you could use much smaller via's. JLCPCB standard is 0.3/0.5mm for a cheap 2 layer board.