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 $9.90   3.181%   $10.2149   $10.21   $10.20   $10.20  
  Purchase #2 $124.48   9.264%   $136.0118   $136.01   $136.00   $136.00  
  Purchase #3 $71.82   3.551%   $74.3703   $74.37   $74.35   $74.40  
  Purchase #4 $21.67   6.481%   $23.0744   $23.07   $23.05   $23.10  
  Purchase #5 $186.91   12.634%   $210.5242   $210.52   $210.50   $210.50  
  Purchase #6 $33.61   12.427%   $37.7867   $37.79   $37.80   $37.80  
  Purchase #7 $117.71   10.153%   $129.6611   $129.66   $129.65   $129.70  
  7 purchase totals = $566.10       $621.6434   $621.63   $621.55   $621.70