Checking memory allocation in Instruments
The ObjectAlloc tool is designed to graph total object allocation over time, and it doesn't take deallocations into account. Essentially: it will always go up.
What you can do is look at the Net Bytes and # Net columns. These columns will adjust based on the number of objects you deallocate, and if you're concerned about total object allocation you can use these to gauge your program's performance.
You can take a look at this thread on the Apple Support forums - it deals more with what Instruments measures and how you can test your app.
You should check out http://developer.apple.com/videos/wwdc/2010/?id=311.
Great video from WWDC 2010 about memory analysis with Instruments.