Q: My Battery says "Service Battery", do I need to replace it?
A: Not necessarily. You can attempt to service the battery by charging to 100% and then discharging to 0%. Sometimes an SMC reset can help. If the message does not go away then its time to assess how long the battery lasts and how long you need it to. Only replace a battery when it no longer lasts long enough for you (or if it is swelling). Not when software tells you to after an arbitrary number of cycles.
Q: My Battery says "Replace Now" or "Replace Soon", do i need to replace it?
A: These messages mean that the battery is still operating normally, but that it holds significantly less charge than when it was new. Its time to assess how long the battery lasts, and how long you need it to. Only replace a battery when it no longer lasts long enough for you. Not when software tells you to after an arbitrary number of cycles.
Q: My Battery is swelling, do I need to replace it?
A: Yes, and sooner rather than later. When a battery starts to swell it will continue to do so, and it can damage your trackpad or your top case. It also carries the small but real risk of fire or serious injury, especially if it is damaged when attempting to remove it.