Reply to: Stylesheet customizationSeptember 18, 2013 at 9:04 pm
You are free to change any part of the toolkit you like (you'll know when you've broken something). However, the DITA OT designers included the custom.xsl file so you can keep all of your overrides and additions in one place, which should make it easier to duplicate your changes to other computers as well as keep track of what has been altered. In theory it might also make it easier to migrate changes to newer versions of the toolkit.
I would recommend that in addition to a comment describing what the change does and why you also include a path to the original file whose template you are overriding, as it will make it easier to find in the future if you need to do so (or if someone else inherits your changes).