Always follow the manufacturers instructions. What would you do if you did have a reaction? You would have no one to blame but yourself.
I have clients all the time who ask me to colour their hair without a skin test, its just not work the risk of someone having a reaction. As a salon we turn away at least 5 people a week as they have not had a skin test, quite often we get tourists requesting the same but its truly not acceptable to apply a tint if they may be allergic.
A quick note regarding the post from tambourine;
Firstly it would be inadvisable to perform a skin test with either cotton wool or Sellotape. The client could have a reaction from both ( get in touch with a leading hair care brand if you need more information on this )
Secondly if the test was to be carried out this way it would state it in the manufacturers instructions!! I would strongly advise from following this method as you would need to check after 24 hours.
Lastly, there would be no benefit carrying out the test if you were only checking against 'developer' this would only prove that you were allergic to 'developer' and not the actual combination of developer and tint. The PPD's that you would be allergic to would be found in the tint section rather than the developer, which is why when in the salon we would use the tint and not the developer.
Hope this helps.
The legal bit:
This is my opinion solely and not the opinion of any of my affiliates or employer.