I have two portable PCs onto which I've installed Leopard. One I've left 10.5.8, and one I've put 10.6.3 onto. The Mini 10v has 10.6.3, and my HP Compaq 6510b has 10.5.8. Why? Simply put, we have Meklort to help us with the Dell Mini's, but there is no such turnkey support for more obscure Hackintoshes. All you have to do to install 10.6.3 on a 10v is to follow the breadcrumbs that have been left on the ground in this forum. But there are enough descriptions of failed installs to lead me to believe that what I see as elementary can be a challenge to others.
For other kinds of hardware, I'm reluctant to tempt fate for not much gain. Truth be told, running Monolingual and StripPPC against 10.5.8 yields pretty much the same hard drive space/speed gain as installing 10.6.3 does. But there are a few features that are better in 10.6.3, especially if you are having trouble with bluetooth or wifi in 10.5.8. Net, I see it kind of like a BIOS upgrade. If you need it, you know it, because something doesn't work. However, if your computer is working fine, why risk some unintentional consequence by installing an upgrade, just because it is "later and greater?"
Net, if you can think of a good reason to upgrade a 10v, and you have an appropriate tolerance for risk and the experience of doing several Mini 9's, go ahead. Netbook Installer 0.8.4 makes upgrading about as risk free as possible. Just keep in mind that this is a production box that your are describing, not a hobby box. If you can't articulate to yourself WHY you want the latest and greatest, then why risk the kind of "oh my gosh, I really, really need this computer for tomorrow morning, and I screwed up the upgrade. Please, please help me!" threads that are unfortunately present elsewhere in this forum?