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 $203.67   12.743%   $229.6237   $229.62   $229.60   $229.60  
  Purchase #2 $130.94   12.258%   $146.9906   $146.99   $147.00   $147.00  
  Purchase #3 $20.79   13.725%   $23.6434   $23.64   $23.65   $23.60  
  Purchase #4 $202.44   10.326%   $223.344   $223.34   $223.35   $223.30  
  Purchase #5 $41.93   4.308%   $43.7363   $43.74   $43.75   $43.70  
  Purchase #6 $204.98   5.847%   $216.9652   $216.97   $216.95   $217.00  
  Purchase #7 $22.61   3.348%   $23.367   $23.37   $23.35   $23.40  
  7 purchase totals = $827.36       $907.6702   $907.67   $907.65   $907.60