If there is an official statement that a certain term should be spelled in a certain way, that definition should be used. E.g. iPod should not be spelled IpOd.
Optionally, if there is no official rule, and if the author is known, you may use the spelling used by the author, as a token of respect for his contribution.
Optionally, you may use the "most usual way" of spelling it, whatever that means.
Actually, if there is no official way of spelling, you can use anything which makes sense.
Practical exception: if some authority mandates a certain spelling, you should probably use that spelling, even if you do not agree, IF it can harm you to do otherwise.
Example: if your university mandates you to spell the iPod as IpOd, then use IpOd - just to be use you did not waste X years of efforts for nothing.