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 $18.75   11.803%   $20.9631   $20.96   $20.95   $21.00  
  Purchase #2 $182.36   4.180%   $189.9826   $189.98   $190.00   $190.00  
  Purchase #3 $184.55   9.410%   $201.9162   $201.92   $201.90   $201.90  
  Purchase #4 $212.50   7.478%   $228.3908   $228.39   $228.40   $228.40  
  Purchase #5 $76.55   3.489%   $79.2208   $79.22   $79.20   $79.20  
  Purchase #6 $71.66   7.572%   $77.0861   $77.09   $77.10   $77.10  
  Purchase #7 $208.32   3.770%   $216.1737   $216.17   $216.15   $216.20  
  7 purchase totals = $954.69       $1,013.7333   $1,013.73   $1,013.70   $1,013.80