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 $12.40   7.118%   $13.2826   $13.28   $13.30   $13.30  
  Purchase #2 $78.45   4.140%   $81.6978   $81.70   $81.70   $81.70  
  Purchase #3 $10.31   3.621%   $10.6833   $10.68   $10.70   $10.70  
  Purchase #4 $211.50   12.578%   $238.1025   $238.10   $238.10   $238.10  
  Purchase #5 $122.26   11.864%   $136.7649   $136.76   $136.75   $136.80  
  Purchase #6 $80.66   5.187%   $84.8438   $84.84   $84.85   $84.80  
  Purchase #7 $51.75   5.318%   $54.5021   $54.50   $54.50   $54.50  
  7 purchase totals = $567.33       $619.8770   $619.86   $619.90   $619.90