I'm also on a 1GB plan and like you I'm blanketed in WiFi. 20 days into my monthly allowance and I just hit 300MBs. My data usage isn't insane, some streaming video, picture uploads, emails, web browsing. Most of which is done on WiFi, so rarely do get close to 1GB.
Several months ago, back when I still had a higher 2GB plan, I turned WiFi off to see how much data I really used. Found out I use about 1.5GBs a month. Knowing that, I felt safe dropping to a 1GB plan.
However, it kills the enjoyment a little bit because I started counting MBs whenever I wasn't on WiFi. I became very hesitant about using data. I quickly found that I wasn't watching videos or loading graphic heavy websites because 1GB isn't a lot. What if I have a few spikes in usage during that month? What if I had to use Nokia/Bing Maps? Mapping data is murder on your data plan.
Do you want to constantly monitor your data 30 days a month? The fear of going over is something to consider. And you may not know how much it's going to bug you until you really know how much data you burn through.
As for me, I'm hopping ship soon to a cheaper bill and higher data cap so I can go back to streaming vids without fear of overages. After all, for as much as I pay, I should be able to use my phone worry free. Doing otherwise kinda defeats the purpose of buying it, right?