Actually you will need shocks any way...
And if you get a TRD
springs and your drop not lower than 2" it's not a big deal if you changhe your shocks or just use your stock shocks.
If you have an Extra money get KYB AGX shock ($400) this shocks are ajstbl.So you can change the stifness of your shocks.
Or you can get a TRD
shocks they are spetially designe for TRD
And reason why people change the shocks is :Yor shocks are bad,yor shocks are old...and bad,You found better shocks,drop of your car will be lower than 2"...
P.S.But you will be fine with the stock shocks if you install TRD