As per the wikis there are crossovers between both the series. So do I have to watch Once Upon a Time in Wonderland to understand Once Upon a Time or can it be avoided fully?
No, the main story arcs of OUaT and OUaTiW are completely independent of each other.
But you might want to watch Frozen (not that I have). I can only assume it lends some character development to these new characters, who seem to have been just dropped in on us. I find the back-story that's given to us in the TV show, lacking.
Side note: you should watch OUaT before you watch OUaTiW, or just know that "magic always comes with a price", that there's no stronger magic then love, and you're good to go.