Per the link above, "The 2008 National Electrical Code®(NEC®) will require new and renovated dwellings to have tamper-resistant (TR) receptacles." My understanding is the state of Washington is using the 2008 NEC. I don't have a newer book here at home so I don't know if the receptacle requirements changed in the 2011 code, in any case it doesn't apply if WA is still using the 2008 version.
The question is whether the garage is considered a dwelling area...if it's an attached garage I think it probably is but if it's a separate structure it may not be, the local inspector may be the best person to ask.
The tamper resistant receptacles aren't that big a deal anyway - they cost about the same as the old style and you can still plug in a power cord like before, they're just designed to keep kids from sticking paper clips in one slot.