Tobias - I'm not sure how true 75 Ohm RCA plug could be but I used Canare 75 Ohm RCA plugs and Canare wire to build my cable. Severity of impedance mismatch, as Almarg explained, depends on rise/fall times and distance. It is common for CD player to have RCA output and for DAC to have BNC input. In such case you have to have either RCA plug or adapter. Get one that is 75 Ohm.
Characteristic impedance at high frequencies depends on ratio of inductance to capacitance (square root to be exact) what translates to geometry of the cable. Same is for RCA plug but dimensions here are already defined and probably not making 75 Ohm. Rest of the plug (body and pin to solder to) might follow 75 Ohm geometry.
Typical CD player has about 25ns rise/fall time. If it's to slow then noise will cause jitter (affecting edges) and if it's too fast (like 5ns) than you'll get reflections on impedance boundaries. Either way it will translate to jitter. My DAC (Benchmark DAC1) is jitter suppressing (upsampling) so cable and transport is not very important to me. Jitter increases noise.
