The mirror or multiple mirrors will affect the quality of your beam transfered from one point to another, but mostly the wavefront quality (read image quality) at the destination. It really depends on what you are trying to accomplish with your testing. For using alignment telescopes or alignment autocollimators, or for critical imaging work, this matters a lot! A poor quality, or consumer grade mirror will cause problems. Avoid a rear-side coated mirror! Always use a front-surface mirror and these will normally be flat mirrors. Here is where it gets interesting.
For good images and optimal alignment (whether by finite focusing telescopes or by autocollimation) expect to use a front surface mirror whose coated surface is at least flat to within 1/4 of a wavelength of green light (say 550 nm or thereabouts). If you have to use multiple mirrors in a single beam steering process, you will be better using flats which are 1/10 of a wave or better.
Remember that the wavefront which bounces off that mirror surface, and is traveling onward will have twice (2X) the wavefront distortion peak-to-valley, as the actual peak-to-valley departure from a perfect plane of that mirror’s surface. Hence a mirror which is really 1/4 wave flat will yield a propagated beam wavefront which is now 1/2 wave.
Optics Lab Tips and Tricks 