Using lots of VM's is one reason to max out the RAM on the new machine, as each VM will require a minimum fixed amount of RAM and you can't change that, but the calculation will be simple - you know how much RAM you need and you wouldn't need to ask. For other applications, like word processing, email, browsing etc., and even some pro level tools such as photo/music/video editing software, the system will just adjust according to usage and page out the inactive ones to swap file. Yosemite is pretty good at this and I still run a 2007 Core2Duo MBP with 4GB of RAM with many of those applications open at the same time and don't notice any disk thrashing. 8GB is plenty enough for most usage now and for the foreseeable future. If you want to max it out at 16GB "just to be safe", then knock yourself out and by all means, it's your money. Who am I to say otherwise.
But know that investing your money in a larger capacity SSD is going to give you far less headache down the road. Yes you can always go for external storage, but having to remember which files were saved to which portable hard disk is difficult, and given Apple's iLife apps and ecosystem, having your music/movies/TV show library and photo library on anything other than your host internal SSD is asking for a lot of inconvenience - iTunes and iPhoto will of course let you choose where you put your libraries, but not having your external drives on hand when you need them is such a hassle, and I speak from experience. You can probably save your space-hogging video files somewhere, but even if you have just a very modest photo library, the giga-bytes will eat up disk space FAST. If you plan on relying on external drives a lot, why not just get a desktop computer like an iMac or something.
