Is toenailing necessary for a joist-to-ledger attachment?
You should always nail your joists to the joist hangers, using the type and quantity of nails recommended by the hanger manufacturer for the type of hangers you're using.  Double-shear hangers, the most common type, will have special nail holes designed to direct the joist nails at an angle through the joist and into the ledger or header.  Typically you'll use 16d or 10d standard-length nails for the joists, and short 1-1/2" 10d nail for fastening the hanger to the ledger or header.  Here's an image from Simpson's website of a properly installed double-shear joist hanger: