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 $39.28   12.377%   $44.1417   $44.14   $44.15   $44.10  
  Purchase #2 $118.72   4.657%   $124.2488   $124.25   $124.25   $124.20  
  Purchase #3 $37.31   3.713%   $38.6953   $38.70   $38.70   $38.70  
  Purchase #4 $39.47   8.587%   $42.8593   $42.86   $42.85   $42.90  
  Purchase #5 $84.62   7.268%   $90.7702   $90.77   $90.75   $90.80  
  Purchase #6 $143.53   9.816%   $157.6189   $157.62   $157.60   $157.60  
  Purchase #7 $48.64   11.481%   $54.2244   $54.22   $54.20   $54.20  
  7 purchase totals = $511.57       $552.5586   $552.56   $552.50   $552.50