It's not necessary to buy new bearing spacers as long as you remember to reuse the spacers that are in your old wheels. Not installing bearing spacers in your new wheels will cause the wheels to spin slowly lock up when the axles are tightened. Most people mistakenly try to solve this by loosening the axles and risk having the wheels come off while they're skating. The most common and dangerous mistake people make when changing their inline wheels is forgetting to install the bearing spacers into the new wheels. If you're buying a set of performance inline wheels with high end bearings, getting a new set of precision aluminum bearing spacers will help get the most free spin (lowest rolling resistance) and is highly recommended. Many times there are other good reasons to buy a new set of spacers:
The spacers that came in your skates are plastic and you want to replace them with a good aluminum set.
You're having difficulty getting your wheels to spin freely when the axles are tightened
You suspect the stock bearing spacers currently installed may be damaged or out of tolerance.
You just don't want to take the time to remove the spacers from the old wheels and would rather just buy a new set.