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 $18.47   9.484%   $20.2217   $20.22   $20.20   $20.20  
  Purchase #2 $12.25   7.498%   $13.1685   $13.17   $13.15   $13.20  
  Purchase #3 $42.73   3.158%   $44.0794   $44.08   $44.10   $44.10  
  Purchase #4 $26.60   3.720%   $27.5895   $27.59   $27.60   $27.60  
  Purchase #5 $29.82   10.678%   $33.0042   $33.00   $33.00   $33.00  
  Purchase #6 $21.66   10.714%   $23.9807   $23.98   $24.00   $24.00  
  Purchase #7 $139.33   12.406%   $156.6153   $156.62   $156.60   $156.60  
  7 purchase totals = $290.86       $318.6593   $318.66   $318.65   $318.70