How subtle or dramatic are the effects of a impedance mismatch by a certain percentage?
You can calculate the reflection coefficient from the well-known formula:

$$\Gamma = \frac{Z_L-Z_0}{Z_L+Z_0}$$

With this you can work out, for example, that a 10% error in the termination impedance gives  about a 5% reflection. 

So really, the question is, how much reflection can you tolerate and still achieve error-free data transfer. And that depends a lot on what other discontinuities exist in your signal path. In source-terminated schemes, for example, we allow near 100% reflections at the load end of the trace, knowing that the termination at the source end will prevent these reflections degrading the signal seen at the receiver. 

To get a good idea how reflections affect your particular situation, you could do a simulation like what's proposed in Rolf Ostergaard's answer to a recent question about SPI.

Generally, just one discontinuity is not likely to cause problems. Multiple discontinuities are more likely to cause problems the farther apart they are on the line because they'll interact at lower frequencies.


  a 50 ohm line with 45 ohm or 55 ohm series termination


Remember that without special attention (and added cost), your board shop will probably only promise a 10% tolerance on the trace characteristic impedance. So if your design calls for a 50 ohm trace, the actual \$Z_0\$ might end up anywhere between 45 and 55 ohms. So a termination that's 10% off of the nominal \$Z_0\$ might end up almost 20% off from the actual \$Z_0\$ (For example a 55 ohm termination would be about 20% mis-matched if the actual trace impedance comes out as 45 ohms.

At modest data rates (say, up to 1 GHz), it's generally easy to ensure that the trace impedance uncertainty is a bigger error than the load mismatch. For example, a 49.9 ohm +/- 1% resistor terminating a 50 ohm +/- 10% microstrip line.