What is a good tolerance for impedance matching or how to calculate losses?
The reflection at the end of a transmission line is given by 

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

Where Z0 is the line's characteristic impedance and ZL is the load impedance.

So if you have a 55 Ohm line and you terminate it with 50 Ohms, you're looking at about 5% reflection. 

If you have a 50 Ohm line, followed by a short length of 55 Ohm line, terminated with 50 Ohms, you're going to get about 5% reflection from the mismatch between the two lines, and 5% reflection from the end termination.

But...these two reflections will interact with each other. Depending on your operating frequency and the length of the mismatched line, they could interfere with each other constructively or destructively. You could have anywhere from 0 to +/-10% reflection from this structure. 

But this is about the voltage signal (or travelling wave) reflection. If you're talking about "losses" you probably want to convert this to power terms. If you know the signal reflection, that would correspond to \$(1-|\Gamma|)^2\$ fraction of the power transmitted to the load. For 10% reflection, that's about 81% power delivery or 0.9 dB of loss.

To find out the exact reflection for your situation, you'd need to either run a simulation, or do a pen-and-paper analysis with a Smith Chart, which we've discussed a little bit in a previous question. The simulator is probably the easier way to go, though, and it will enable you to do things like sensitivity analysis more quickly.