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 $129.96   4.857%   $136.2722   $136.27   $136.25   $136.30  
  Purchase #2 $174.74   8.121%   $188.9306   $188.93   $188.95   $188.90  
  Purchase #3 $81.32   4.178%   $84.7175   $84.72   $84.70   $84.70  
  Purchase #4 $236.27   3.658%   $244.9128   $244.91   $244.90   $244.90  
  Purchase #5 $218.53   3.165%   $225.4465   $225.45   $225.45   $225.40  
  Purchase #6 $96.72   4.784%   $101.3471   $101.35   $101.35   $101.30  
  Purchase #7 $14.39   3.835%   $14.9419   $14.94   $14.95   $14.90  
  7 purchase totals = $951.93       $996.5686   $996.57   $996.55   $996.40