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 $13.68   11.383%   $15.2372   $15.24   $15.25   $15.20  
  Purchase #2 $7.98   8.620%   $8.6679   $8.67   $8.65   $8.70  
  Purchase #3 $128.42   13.714%   $146.0315   $146.03   $146.05   $146.00  
  Purchase #4 $52.83   10.262%   $58.2514   $58.25   $58.25   $58.30  
  Purchase #5 $226.72   4.610%   $237.1718   $237.17   $237.15   $237.20  
  Purchase #6 $51.63   3.430%   $53.4009   $53.40   $53.40   $53.40  
  Purchase #7 $110.26   9.157%   $120.3565   $120.36   $120.35   $120.40  
  7 purchase totals = $591.52       $639.1172   $639.12   $639.10   $639.20