Yes you beed to make a bleed as long as you need to print it in offset or digital print.
If you forgot making a bleed you can add it by going to File> Document Setup and enter you 0.3 cm bleeds.
Thank you Omar!
If for example, I want to make an A4 poster, should I make the size A4 inclusive of 0.3cm bleeding or should I do A4+ 0.3cm bleeding?
Usually the bleed adds to the end format.
But please ask the printing service for specifications.
Some want you to add bleed.
Some want you to set up the file slightly larger, so the document dimensions already include the bleed.
Some want 2mm, some 3, some even just 1. Depens on the service provider. Talk to them.
"was wondering whether I should design a template and apply bleed lines or is this not necessary?"...
You only need a bleed if a background element extends beyond your trim. If you plan on printing via a desktop printer, then a bleed only works if you print on a larger size and trim. Not sure what you mean by "template"? A template would be used if you plan on creating future designs based on the one you are creating now.
Use "BLEED" for professional printing processes.
Use Crimean guides with Bleed.
You can use a template. As long as the same measures are the same cuts.