Signal velocity in PCB traces
The rules of thumb given in the previous answer are good enough for many designs. But I want to add one additional thought.

The velocity factor is basically going to be the inverse square root of dielectric constant (\$\varepsilon_R\$ or \$D_k\$) of the material the electric field around the transmission line travels through. 

For stripline, this means it's essentially the dielectric constant of the circuit board material.

But for microstrip, it will be an average of the board material's dielectric constant, and the surrounding material (usually air), weighted by the proportion of the electric field that travels in each medium.

That means that if your design has both microstrip and stripline, signals in the microstrip will travel at least slightly faster than signals in the stripline.

As pointed out in the comments, it's worth mentioning that the dielectric constant of PWB material can vary in service due to factors such as operating frequency, temperature change and moisture absorption. There can also be variation at manufacturing time due to factors such as etch-back of the traces and alignment of the traces relative to the glass fibers in the dielectric.


  If I have a trace that has a width change, then there will be a characteristic impedance change, and that would cause partial reflection of the incoming wave. But there is no energy absorption anywhere, so the only cause for a partial reflection can be a change in the wave's propagation speed, right?


No, that doesn't follow.

If there's a discontinuity in the characteristic impedance, then you need a different ratio of current to voltage in the line for the incoming signal and the forward-propagating signal. That means to satisfy KCL and KVL (or, if you want a more mathematical explanation, to satisfy the boundary conditions) at the place the two geometries meet, a reverse travelling wave has to be generated. Then the current to voltage ratio on the incoming line at the junction can be made to match that on the outgoing line, and all is right with the universe.

The difference in propagation velocity on the two lines, if there is one, isn't important here.