I used to do freelance PCB design so was pretty well into it (anyone remember Bishop graphics and DOS Protel?), but I'm currently doing a board after quite some time off (maybe 20 years) using SMDs and 4 layers and it seems the rules have changed a bit.
In the past you would religiously run one side vertical and the other side horizontal. The second you snuk a horizontal trace vertically (or vv) by even a few 10ths you would cut your water off by blocking a future trace. At the end you could rip up and re-route when you had no more traces to run but that rule was absolute until then.
Now it seems all different. Many SMDs don't allow traces between pins so they often totally block a large area of PCB on one side. For example a the SAM3U, 144 pins all of which have to be broken out with no possibility of routing between them. So regardless of where they are going they all have to start on the component side.
In other places there are components on both sides so all bets are off and you have to be careful not to really box yourself in.
Now I find myself just running traces on whatever side gets them through and to hell with the rules.
On the plus side is 4 layers. What a breath of fresh air to not have to find space for large power traces. Just a short stub off a pin and a via to a middle layer. Great stuff.
Anyway just a couple of thoughts about the current state of the art as I see it.
Is it fair to say that the old horizontal/vertical rule no longer applies (or at least not as much)?