Depends on the size of your company, staff allocated to research level of positions or job functions etc.
We use an agency wide approach with a 3-4 year rotation cycle.
General User:
-We choose a standard type of pc. Ex: Dell w/3-4 year warranty
-We request 2-loaner pc's in advance, based on specific specs to last the 3-4 years, build our standard image and test
-We then order 1/3 of the staff total worth of computers 2 or 3 times a year. While trying to stay within the same pc family in order to minimize our baseline images.
-We are in the position that we no longer have to worry too much about the job function when assigning new equipment.
-We deploy hardware based on older hardware that have or will fall out of warranty. Therefore, out of the rotation cycle.
-We reserve X amount of pc?s for those higher demanding positions or projects and use as needed or for potential new staff.
-Equipment being returned is prepared for surplus to other agencies or schools.
Power Users:
-We order a higher-class pc.
-Deployed based on power user justification
Traveler:
-Since the cost for laptops is higher than a pc. We?re more conservative here. We upgrade as needed only.
-Deployed based on travel justification
-Users are docked w/no extra monitor. Policy for travelers is laptop or pc but not both.
Our standards have proven to work smoothly.