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 $113.90   6.463%   $121.2614   $121.26   $121.25   $121.30  
  Purchase #2 $55.65   3.696%   $57.7068   $57.71   $57.70   $57.70  
  Purchase #3 $170.66   10.341%   $188.308   $188.31   $188.30   $188.30  
  Purchase #4 $167.79   3.525%   $173.7046   $173.70   $173.70   $173.70  
  Purchase #5 $175.55   6.635%   $187.1977   $187.20   $187.20   $187.20  
  Purchase #6 $246.93   13.345%   $279.8828   $279.88   $279.90   $279.90  
  Purchase #7 $239.73   5.560%   $253.059   $253.06   $253.05   $253.10  
  7 purchase totals = $1,170.21       $1,261.1203   $1,261.12   $1,261.10   $1,261.20