Read the technical reason why material documents are not checked in OSS note 547867 - FAQ: MM_MATNR Archiving of materials
Nevertheless there is certainly an impact if you remove materials from the data base but still have material documents.
First of all they are a kind of orphaned after the archiving of the materials. They are in the MKPF/MSEG table and maybe included in some reports but you wont see a material description which makes the information useless and disturbing.
While I can understand the technical reason which is given in the OSS note (one material document can have several lines for different materials) I still the opinion it is a kind of theoretical. The material master is from my point of view one of the last archiving objects that can be executed because of it long chain of dependencies. And the material documents are the most easy straight forward archiving object that exists in SAP. No dependencies at all.
Anything is accessible in the archive, so the usual way of archiving is to archive those object that are easy to archive and are very large objects which bring a benefit immediately are the first who are executed. So you rarely find a case in reality where you have movements when you archive materials.
you should search and read all archiving object related OSS notes before you archive, it is really giving knowledge and safety.
Even you want prepare yourself before you include keyusers you should not hesitate to contact them already occasionally to speed up your process. A telephone call to colleague is much quicker than a discussion in SCN which spreads over more than a week