Is the speed of your bow a good thing to know? It could be, if you checked it often. It would let you know if something was wrong with your bow or something if your speeds started to change, but there are other ways to determin this.
I think there are two things some people should not know about there bows. One is the speed they are shooting, the other is the poundage they draw. The only reason you should need to know your poundage is to make sure you get the proper arrows. Other than that it will just mess with your head. To many people draw more weight than they should because they want a faster bow. If you didn't know the speed, or the draw weight you wouldn't be concerned with it and your bow would most likely fit you better. Just my opinion any way.
When we refer to Grain weight and it is above 200 grns (grains) we are usually talking about finnished arrows. That includes the tip (field tip or broad head, they should be the same weight), arrow shaft, and fletchings. If it is 75 grns to 150 grns we are usually talking about a tip or broad head. The most common is 100 and 125 grns.
Normally arrows now a days are going to be between 300 and 500 grns, with the norm around 350 to 400 grns I would guess. I could be wrong on that though, someone should do a poll on the average arrow weight or something. 300 grns would be pretty light for most average set ups. The reason we concern ourselves with it is because most bows are waranteed for 5 grns per lb of draw weight. If you are drawing 70 lbs your arrow should weigh no less than 350 gns, or you may cause harm to your bow from shooting too light of an arrow.
Actually there is more to it than just draw weight. Your draw length and how effecient your bow is also come into play. They just base it on a 30 inch draw length. If you have a shorter draw length like I do (26 inches) you can get away with shooting a bit lighter arrow because your bow does not produce as much energy as one with a 30 inch draw length. There is a minimum weight AMO chart on Bowjacksons sight.
http://home.att.net/~sajackson/archery.html
Hear is another link for you to check out.
http://www.huntersfriend.com/bowselection.htm
Lots of explinations on this site for you.
Let me know if you want some more links for archery info.
Paul