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 $36.71   12.493%   $41.2962   $41.30   $41.30   $41.30  
  Purchase #2 $116.94   11.331%   $130.1905   $130.19   $130.20   $130.20  
  Purchase #3 $135.48   3.117%   $139.7029   $139.70   $139.70   $139.70  
  Purchase #4 $175.73   10.436%   $194.0692   $194.07   $194.05   $194.10  
  Purchase #5 $23.61   10.135%   $26.0029   $26.00   $26.00   $26.00  
  Purchase #6 $42.89   5.402%   $45.2069   $45.21   $45.20   $45.20  
  Purchase #7 $21.34   7.704%   $22.984   $22.98   $23.00   $23.00  
  7 purchase totals = $552.70       $599.4526   $599.45   $599.45   $599.50