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 $34.67   9.133%   $37.8364   $37.84   $37.85   $37.80  
  Purchase #2 $108.89   4.799%   $114.1156   $114.12   $114.10   $114.10  
  Purchase #3 $234.61   5.148%   $246.6877   $246.69   $246.70   $246.70  
  Purchase #4 $156.33   4.827%   $163.876   $163.88   $163.90   $163.90  
  Purchase #5 $46.93   10.677%   $51.9407   $51.94   $51.95   $51.90  
  Purchase #6 $68.92   7.778%   $74.2806   $74.28   $74.30   $74.30  
  Purchase #7 $113.86   13.256%   $128.9533   $128.95   $128.95   $129.00  
  7 purchase totals = $764.21       $817.6903   $817.70   $817.75   $817.70