You can monkey patch Float to do this, basically.  Whether or not it's
a good idea is another matter entirely.

class Float
  def to_s
    #force all Floats to 2 decimal places
    "%.2f" % self
  end
end

puts 1.2345678  # => 1.23

Or maybe:

class Float
  attr_accessor :decimals
  alias_method :default_to_s, :to_s
  def to_s
    decimals ? "%.#{decimals}f" % self : default_to_s
  end
end

a = 1.234567
puts a   # => 1.234567
a.decimals = 2
puts a   # => 1.23