Are your boards standing above the surface of the rink?  Are they shorter than four feet?  You may want to pad them with something in the event someone falls on or near the boards.

Here are some suggestions... 

NiceRink Bumper Caps
NiceRink has some great products and they all work great, so as I have said in the past "If money isn't an issue, get these!"  These also help to hold the liner in place making it so you do not have to put holes in your liner.

Pipe Insulation
You can find this at any home improvement store.  It probably won't be as thick as the NiceRink Bumper Caps, but will do the job.

Swimming Pool Noodles
The ones that have the small hole in the middle.  Cut a slit down one side of them and spread them over your boards.  These also help to hold the liner in place making it so you do not have to put holes in your liner.

I picked up 30 pool noodles this year and am going to give them a try this season (2007-2008), They were on sale and were worth a try, I'll report back on them during/after the season.