Are there any disadvantages to unusual RAM amounts for virtual machines?
Solution 1:
I've never ever seen any proof that the precise value of RAM matters on a virtual machine. I suppose it's not implausible that some legacy systems may somehow expect an even number, but that would just be incredibly bad engineering.
Certainly - it's pretty natural for people to allocate in multiples, but this tends to be because they know Server [A] needs 8GB of RAM, rather than 7GB.
If you want to give you server 3GB, then absolutely go right ahead; I use odd numbers all of the time.
Solution 2:
It's fine. There's no downside. Remember, the granularity is is Megabytes, so if you need 2,560MB instead of 2GB or 3GB, it's fine, fully supported and a non-issue.