default ordering of query result - are they always guarantee

I have the following table :

CREATE TABLE measurement_1
(
measurement_id serial NOT NULL,
fk_unit_id int NOT NULL,
"value" double precision,
measurement_type text NOT NULL,
measurement_unit text NOT NULL
);

When I want to retrieve the query. By default, the query result are ordered in ascending order, by using measurement_id.

SELECT measurement_type, value, measurement_unit
FROM
measurement_1

This is the default behavior I want. However, I am not sure whether this is always guarantee? Or shall I explicitly make the query in the following form? Will this have performance impact on row with millions?

SELECT measurement_type, value, measurement_unit
FROM
measurement_1 ORDER BY measurement_id ASC

Thanks.

by ensuring measurement_1 result are ordered in ascending
Thanks and Regards
Yan Cheng CHEOK