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 $18.56   13.652%   $21.0938   $21.09   $21.10   $21.10  
  Purchase #2 $20.91   6.518%   $22.2729   $22.27   $22.25   $22.30  
  Purchase #3 $187.96   4.478%   $196.3768   $196.38   $196.40   $196.40  
  Purchase #4 $106.84   12.488%   $120.1822   $120.18   $120.20   $120.20  
  Purchase #5 $84.84   11.817%   $94.8655   $94.87   $94.85   $94.90  
  Purchase #6 $139.61   5.112%   $146.7469   $146.75   $146.75   $146.70  
  Purchase #7 $182.47   13.429%   $206.9739   $206.97   $206.95   $207.00  
  7 purchase totals = $741.19       $808.5120   $808.51   $808.50   $808.60