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 $10.91   13.803%   $12.4159   $12.42   $12.40   $12.40  
  Purchase #2 $120.93   6.849%   $129.2125   $129.21   $129.20   $129.20  
  Purchase #3 $99.57   13.780%   $113.2907   $113.29   $113.30   $113.30  
  Purchase #4 $202.59   8.167%   $219.1355   $219.14   $219.15   $219.10  
  Purchase #5 $24.92   5.360%   $26.2557   $26.26   $26.25   $26.30  
  Purchase #6 $190.88   4.487%   $199.4448   $199.44   $199.45   $199.40  
  Purchase #7 $129.45   9.834%   $142.1801   $142.18   $142.20   $142.20  
  7 purchase totals = $779.25       $841.9352   $841.94   $841.95   $841.90