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 $36.99   6.794%   $39.5031   $39.50   $39.50   $39.50  
  Purchase #2 $238.26   12.762%   $268.6667   $268.67   $268.65   $268.70  
  Purchase #3 $12.79   8.467%   $13.8729   $13.87   $13.85   $13.90  
  Purchase #4 $127.76   10.362%   $140.9985   $141.00   $141.00   $141.00  
  Purchase #5 $94.51   9.682%   $103.6605   $103.66   $103.65   $103.70  
  Purchase #6 $11.44   11.787%   $12.7884   $12.79   $12.80   $12.80  
  Purchase #7 $246.39   10.835%   $273.0864   $273.09   $273.10   $273.10  
  7 purchase totals = $768.14       $852.5765   $852.58   $852.55   $852.70