To quarantine or not is a personal choice you have to make based on your own evaluation of the risks.
Would you be very upset if you got pond snails from the plant? do you have expensive fish that youíd be devastated to see get sick or you donít want to medicate your main tank? Best to quarantine. Do you not have much in your main tank; thoroughly trust your lfs
or wouldnít mind medicating the whole tank if something comes up? then skip it.
I almost always quarantine everything but Iím only looking for the most common of issues so I may only do a week or so. Itís insufficient time for me to be absolutely sure the fish are free of everything but itís a level of risk Iím usually willing to accept. For plants I usually rinse well in tap water then put them in a bucket for a few dayís just to look for snails on the side of the bucket. Again; insufficient to completely ensure no hitchhikers but itís the level of risk Iíve found Iím okay with.
The proper quarantine technique if you really REALLY want to protect your main tank is the new fish spend months on their own then an inhabitant from the main tank moves into the quarantine tank to check for diseases the new fish may be asymptomatic for. As you can imagine very few people are doing quarantine with that level of rigor, instead people determine what level of risk they are willing to accept.