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 $15.39   7.213%   $16.5001   $16.50   $16.50   $16.50  
  Purchase #2 $237.93   5.660%   $251.3968   $251.40   $251.40   $251.40  
  Purchase #3 $145.49   9.259%   $158.9609   $158.96   $158.95   $159.00  
  Purchase #4 $239.37   12.761%   $269.916   $269.92   $269.90   $269.90  
  Purchase #5 $205.72   3.487%   $212.8935   $212.89   $212.90   $212.90  
  Purchase #6 $237.37   11.241%   $264.0528   $264.05   $264.05   $264.10  
  Purchase #7 $116.70   5.420%   $123.0251   $123.03   $123.05   $123.00  
  7 purchase totals = $1,197.97       $1,296.7452   $1,296.75   $1,296.75   $1,296.80