Based on what you have said I would assume that if User 1 fills out the form and submits/saves it, if User 2 opens User 1's form it will trigger your rule again, because unless you have code in the form or a control that you are using to indicate that the form was already created it will be triggered again. Becasue of this User 2's name not User 1's will be displayed.
Now if there is never an instance where a different user will open User 1's form, ie the form is meant to be filled out and then printed, then obviously this is not an issue. However if this form is meant to be filled out and then submitted to a someone, like a superviosr for approval, then the rule will be triggered and enter the new User's name as the current user. Test this by creating a form and saving it and then having a coworker open it.
I brought this up because I had the problem of users changing when I first implemented the use of a webservice to pull the current users information, however instead of using a on open rule, I simply set the default value of the text box to the Users Name, using the method that you linked to in your first post, and unchecked the "Update this value..." option, this way the text boxes default value only fired the first time the form was opened.