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 $17.99   3.750%   $18.6646   $18.66   $18.65   $18.70  
  Purchase #2 $159.40   12.761%   $179.741   $179.74   $179.75   $179.70  
  Purchase #3 $12.71   11.582%   $14.1821   $14.18   $14.20   $14.20  
  Purchase #4 $67.38   11.158%   $74.8983   $74.90   $74.90   $74.90  
  Purchase #5 $163.58   7.241%   $175.4248   $175.42   $175.40   $175.40  
  Purchase #6 $165.68   5.806%   $175.2994   $175.30   $175.30   $175.30  
  Purchase #7 $246.51   13.648%   $280.1537   $280.15   $280.15   $280.20  
  7 purchase totals = $833.25       $918.3639   $918.35   $918.35   $918.40