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 $229.82   13.359%   $260.5217   $260.52   $260.50   $260.50  
  Purchase #2 $166.80   10.267%   $183.9254   $183.93   $183.95   $183.90  
  Purchase #3 $102.36   10.175%   $112.7751   $112.78   $112.80   $112.80  
  Purchase #4 $246.91   4.816%   $258.8012   $258.80   $258.80   $258.80  
  Purchase #5 $90.56   11.622%   $101.0849   $101.08   $101.10   $101.10  
  Purchase #6 $82.66   4.298%   $86.2127   $86.21   $86.20   $86.20  
  Purchase #7 $74.53   4.704%   $78.0359   $78.04   $78.05   $78.00  
  7 purchase totals = $993.64       $1,081.3569   $1,081.36   $1,081.40   $1,081.30