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 $37.30   7.203%   $39.9867   $39.99   $40.00   $40.00  
  Purchase #2 $102.92   7.526%   $110.6658   $110.67   $110.65   $110.70  
  Purchase #3 $120.29   7.430%   $129.2275   $129.23   $129.25   $129.20  
  Purchase #4 $175.79   8.161%   $190.1362   $190.14   $190.15   $190.10  
  Purchase #5 $188.82   13.729%   $214.7431   $214.74   $214.75   $214.70  
  Purchase #6 $95.56   13.355%   $108.322   $108.32   $108.30   $108.30  
  Purchase #7 $64.51   8.755%   $70.1579   $70.16   $70.15   $70.20  
  7 purchase totals = $785.19       $863.2392   $863.25   $863.25   $863.20