Another option is not to modify web.config, but to modify master page. Every SharePoint master page contains ScriptManager tag. It has ScriptMode attribute that is Auto by default (or if not presented). You can set it up to Debug.
Result will be the same as for web.config, but only for one particular site, where you customize master page. This should be sufficient, but if it is not working, try also modify web.config. And, of course, do it only on developer machine.
Hope this tip helps and as usual good luck in spdevelopment!