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 $73.35   10.178%   $80.8156   $80.82   $80.80   $80.80  
  Purchase #2 $37.59   3.807%   $39.0211   $39.02   $39.00   $39.00  
  Purchase #3 $180.49   11.418%   $201.0983   $201.10   $201.10   $201.10  
  Purchase #4 $115.27   12.858%   $130.0914   $130.09   $130.10   $130.10  
  Purchase #5 $13.61   11.797%   $15.2156   $15.22   $15.20   $15.20  
  Purchase #6 $49.83   3.819%   $51.733   $51.73   $51.75   $51.70  
  Purchase #7 $118.64   5.832%   $125.5591   $125.56   $125.55   $125.60  
  7 purchase totals = $588.78       $643.5341   $643.54   $643.50   $643.50