Looks like some people think otherwise:
"22. Will my neodymium magnets lose strength over time?
Very little. Neodymium magnets are the strongest and most permanent magnets known to man. If they are not overheated or physically damaged, neodymium magnets will lose less than 1% of their strength over 10 years - not enough for you to notice unless you have very sensitive measuring equipment. They won't even lose their strength if they are held in repelling or attracting positions with other magnets over long periods of time.
23. Will neodymium magnets lose strength if they are held in repelling or attracting positions for a long time?
In most applications, the answer is simply "no". If the magnets will be exposed to higher temperatures while in repelling applications, the answer is "possibly". The exact answer is a bit too complicated for a FAQ answer, and requires specifics about the application."
From the FAQs at a leading manufacturer of magnets:
http://www.kjmagnetics.com/faq.asp#timeOf course I suppose they could be wrong.
I wonder who I should believe?
Force-distance relation and properties of repelling Sm-Co5 magnets in orthodontic clinical use: an experimental model
Lars Bondemark 1 Jüri Kurol 1
1 Department of Orthodontics, Institute for Postgraduate Dental Education, Jönköping, Sweden
Correspondence to J. Kurol, Department of Orthodontics, Institute for Postgraduale Dental Education, Järnvägsgatan 9, S-553 15 Jänköping, Sweden
Copyright Munksgaard 1992
KEYWORDS
dental alloys • magnetics • orthodontic appliances • stress, mechanical
ABSTRACT
In recent years, magnets and magnetic forces have been suggested as an alternative to traditional orthodontic devices such as elastics, springs and wires. The purpose of this study was to analyze the reliability and the Output of forces for prefabricated orthodontic repelling Sm-Co5 magnets and the force conditions for the magnets after 5 months of maximal loading. The test-machine consisted of a jig where the tested pair of repelling magnets was mounted close to the clinical situation. With a cylindrical strain gauge transducer, a micrometer screw, a bridge-amplifier, and a Mingograph jet recorder, force-distance diagrams were constructed. The force-distance diagrams showed that the force–distance curve was hyperbolic and that the mean force when the magnetic pole faces were almost in contact with each other was 214. 9 g, SD 13. 42 g. The Variation of magnetic force between different Sm-Co5 magnets was 6–9%. The difference in force before and after 5 months of maximal loading of the magnets was not significant. The test results indicate that with proper handling of the forces according to the new force diagrams, the system is reliable for orthodontic use.
