Someone setting a (e-mail) tab to 5 spaces is asking for horror. They deserve horror.

IMHO anyone setting a TAB to something other than 8 spaces should not send patches or be able to commit to a central repo.

I have no strong feelings for tab vs spaces. As long as they are used consistently within the team (teams are huge for open source projects), it is fine with me.

I much more care about indentation. Specifically with people that use cut-n-paste from other files and do not adjust it for the current file. Oh horror!