Should I erase my hard drive before updating to Yosemite?

I am planning on updating to OS X 10.10 Yosemite on my late 2009 3GHz 4GB RAM iMac soon from 10.9.5 Mavericks, and I've always heard it is best to erase the hard drive and wipe it clean when installing a new version of the OS.  Is this still true? 
And since the OS no longer comes on an install CD disc, is it still recommended to erase the HD before installing fresh?  If so, how is this best accomplished without such an install CD?  Copy the downloaded installer to a flash drive or CD/DVD or external hard disk?
Or has the Mac OS progressed enough that simply using Software Update is now the best and recommended way to upgrade OS X?
Any best practice recommendations would be most appreciated.  And as always, I backup all of my important stuff before I perform such an install (though I do not use Time Machine as yet).

You don't say how you are backing up, other than that you don't use Time Machine (which is not a bad thing, given the reports of problems with the reliability of Time Machine backups). I'd suggest that you buy Carbon Copy Cloner and use it to make a complete, bootable backup of your system on a second hard drive.
4GB of RAM is borderline skimpy. You may need to at least double what you have installed now: http://eshop.macsales.com/shop/memory/iMac/DDR3_21.5_27
Download DiskmakerX: http://diskmakerx.com/, and get an 8GB flash drive. You can use DiskmakerX to put the Yosemite installer on the flash drive.I'd also suggest that you try to re-download the Mavericks installer before you do any other steps, and store it in a safe location outside of your Applications folder. You may want to revert or re-install at some point. You can also make a flash drive for Mavericks, if you have succeeded in downloading it.
"Install OS X Yosemite.app" (or or any installer later than Snow Leopard) is downloaded to your Applications folder. When the download is complete, the installer application launches. It is at this point that you quit the installer and copy (not move) it to a safe location outside of your Applications folder, perhaps in the folder of backups you (should) maintain for stuff like system updates and installers. You need to do this is because "Install OS X Whatever.app" very unhelpfully deletes itself after the installation is complete.
Make your flash drive installer at this point. When you are done, locate the copy of the Yosemite installer that is still in your Applications folder, launch it, and upgrade the system on your internal drive. With a cloned, bootable backup, you can easily revert to Mavericks if Yosemite doesn't run well.

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