A basic fact of riding is that wheel spokes slowly become loose. They also tend to become loose unevenly so certain areas of the wheel become weaker than others. You can however do a decent job of wheel maintenance, even without a truing stand. It's amazing how often we find floppy spokes in our wheels so it's always worth having a spoke wrenc close by.

This Basic Spoke Wrench is compact in size and will work on three different nipple sizes: 3.2mm, 3.3mm and 3.5mm. It is a very useful tool to have in your pack or car glove-box. 

Note that this spoke wrench engages on two sides of the spoke nipple, whereas some other spoke wrenches engage on three sides of the nipple. If you are building wheels, or doing regular truing using a stand - it would be worth investing in a tool with three side engagement. We sell tools like this here. It would still be worth having this tool for your pack though, because those are a little larger and only fit one size of spoke.