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 $100.60   13.183%   $113.8621   $113.86   $113.85   $113.90  
  Purchase #2 $240.57   3.715%   $249.5072   $249.51   $249.50   $249.50  
  Purchase #3 $72.72   10.689%   $80.493   $80.49   $80.50   $80.50  
  Purchase #4 $152.96   11.608%   $170.7156   $170.72   $170.70   $170.70  
  Purchase #5 $16.90   7.172%   $18.1121   $18.11   $18.10   $18.10  
  Purchase #6 $172.62   5.416%   $181.9691   $181.97   $181.95   $182.00  
  Purchase #7 $102.93   3.633%   $106.6694   $106.67   $106.65   $106.70  
  7 purchase totals = $859.30       $921.3285   $921.33   $921.25   $921.40