As long as your client will never want an attribute that is added to the base price for any product...

"but then some product have attributes and some don't "
Why do you think the suggestion will cause any product not to have attributes? Being default does not remove the attribute from displaying as such; it is merely pre-selected. Have you tried this?

I just hope you have made good notes and/or saved backup copies so your changes don't get wiped out in upgrades (especially if done by someone else in the future). Most if not all of the files you edited are not at this point part of the template override system, so there is not a standard method to carry on changes.