Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $90.72   10.254%   $100.0224   $100.02   $100.00   $100.00  
  Purchase #2 $28.98   3.340%   $29.9479   $29.95   $29.95   $29.90  
  Purchase #3 $240.27   4.425%   $250.9019   $250.90   $250.90   $250.90  
  Purchase #4 $45.31   8.605%   $49.2089   $49.21   $49.20   $49.20  
  Purchase #5 $143.32   4.439%   $149.682   $149.68   $149.70   $149.70  
  Purchase #6 $227.88   12.678%   $256.7706   $256.77   $256.75   $256.80  
  Purchase #7 $18.77   8.517%   $20.3686   $20.37   $20.35   $20.40  
  7 purchase totals = $795.25       $856.9023   $856.90   $856.85   $856.90