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 $19.90   8.222%   $21.5362   $21.54   $21.55   $21.50  
  Purchase #2 $56.72   9.195%   $61.9354   $61.94   $61.95   $61.90  
  Purchase #3 $80.38   3.553%   $83.2359   $83.24   $83.25   $83.20  
  Purchase #4 $34.50   7.582%   $37.1158   $37.12   $37.10   $37.10  
  Purchase #5 $45.35   7.420%   $48.715   $48.72   $48.70   $48.70  
  Purchase #6 $244.44   10.532%   $270.1844   $270.18   $270.20   $270.20  
  Purchase #7 $30.91   11.751%   $34.5422   $34.54   $34.55   $34.50  
  7 purchase totals = $512.20       $557.2649   $557.28   $557.30   $557.10