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 $18.31   9.595%   $20.0668   $20.07   $20.05   $20.10  
  Purchase #2 $101.49   9.305%   $110.9336   $110.93   $110.95   $110.90  
  Purchase #3 $234.71   13.261%   $265.8349   $265.83   $265.85   $265.80  
  Purchase #4 $234.72   4.247%   $244.6886   $244.69   $244.70   $244.70  
  Purchase #5 $228.73   7.759%   $246.4772   $246.48   $246.50   $246.50  
  Purchase #6 $81.31   3.700%   $84.3185   $84.32   $84.30   $84.30  
  Purchase #7 $247.52   13.228%   $280.2619   $280.26   $280.25   $280.30  
  7 purchase totals = $1,146.79       $1,252.5815   $1,252.58   $1,252.60   $1,252.60