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 $150.43   6.409%   $160.0711   $160.07   $160.05   $160.10  
  Purchase #2 $114.94   5.152%   $120.8617   $120.86   $120.85   $120.90  
  Purchase #3 $237.49   12.137%   $266.3142   $266.31   $266.30   $266.30  
  Purchase #4 $25.63   5.862%   $27.1324   $27.13   $27.15   $27.10  
  Purchase #5 $26.60   4.448%   $27.7832   $27.78   $27.80   $27.80  
  Purchase #6 $48.33   8.729%   $52.5487   $52.55   $52.55   $52.50  
  Purchase #7 $14.94   6.370%   $15.8917   $15.89   $15.90   $15.90  
  7 purchase totals = $618.36       $670.6030   $670.59   $670.60   $670.60