Does anyone think readability is affected when a method is called without parentheses in Ruby

Answer 1

Yes, leave them in.

Answer 2

Depends.

I don't use parentheses usually. Ruby is beautiful enough and doesn't hamper the readability in most cases.

Example,

def sum a,b
a+b
end

sum 1,2 # allowed, doesn't seem as a hassle.

But, there are some cases you definitely should use parentheses.
eg, if the method takes quite a number of arguments,
or if you're printing the return value

puts sum 1,2 # discouraged

You should read the ruby style guide, as it's designed for maximum efficiency.
https://github.com/bbatsov/ruby-style-guide