Most of the time when we are told to stand up straight relaxing is not going through our heads. This is usually someone telling us to improve our posture or stand at attention. In these particular situations the body goes rigid and upright. For fencing the two need to be accomodated in order to achieve the most effective on guard, or ward position.
Being rigid in the on guard position is detrimental to your fencing. When you are rigid, your muscles are already burning energy and are already tensed. This means that they are not ready for movement which leads you to slower movement, which can decide whether you are struck or not. In order to fight this you need to relax your body, so only those muscles that need to be working are actually working.
Standing up straight means that you are standing tall. Your chest is expanded and you have an air of confidence about your stance. Both of these elements are important in the on guard stance. With the chest expanded it is much easier to breathe, this means you have more energy due to the increase in breath. Your muscles are also not tensed as much if you were slouching, this goes especially for those which are over the shoulders.
So, the trick is to combine the relaxed but upright position into the on guard position. This may sound like a contradiction, but it is not. First of all, spread your feet to shoulder width, remember to keep the front foot pointed at the opponent. Bend your knees somewhat, but not so much that they become tensed. You should still be able to move your feet easily. Your body should be in an upright position, with your spine vertical. Push your chest out, and roll your shoulders down and in. This should expand your chest and make it easy to breathe. Keep your head upright. Now, breathe deeply in and hold it, then let it out slowly. Do this a couple of times and mentally relax all of your muscles. What you should find as a result is that the body is relaxed and ready for action and is also upright.
The common mistake that is made in the on guard position is that the shoulders are slouched forward. This pushes the shoulders forward, and also the head forward. The position that results also constricts the breathing of the fencer and makes it more difficult for them to breathe. Standing in this manner also tenses muscles, tightening them and making it harder to move. This is usually found in beginners, or fencers who face up against a more experienced opponent. What they are tying to do is shrink themselves into a smaller package and hide. Needless to say, this does not result in good fencing.
Stand up straight in your on guard stance, you will be able to breathe better and move more efficiently. The other thing is that standing up straight gives you an air of confidence and makes you feel more confident, neither of which is a bad thing in fencing. A relaxed but upright position is advantageous for all the reasons above, but the same principles can be applied to any on guard position that is found in fencing. Expand the chest, keep the head up and relax.