can it harm my system to block all traffic

[Miike]

When I set up a Firewall (iftables) the guides say to start to block
everything by default and after that allow the stuff that needs to be
allowed.

My question is:
Can it harm my system to deny everything in the first step ?
Since some components are internaly communicating with each other with
a network connection. (Loopback device) and so on...

Or doesnt my system care when I deny everything?

Thanks.

[kallamej]

Just add the rule