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 $13.96   10.335%   $15.4028   $15.40   $15.40   $15.40  
  Purchase #2 $189.64   5.799%   $200.6372   $200.64   $200.65   $200.60  
  Purchase #3 $110.40   11.258%   $122.8288   $122.83   $122.85   $122.80  
  Purchase #4 $45.67   12.582%   $51.4162   $51.42   $51.40   $51.40  
  Purchase #5 $110.53   9.774%   $121.3332   $121.33   $121.35   $121.30  
  Purchase #6 $14.97   9.367%   $16.3722   $16.37   $16.35   $16.40  
  Purchase #7 $132.40   3.377%   $136.8711   $136.87   $136.85   $136.90  
  7 purchase totals = $617.57       $664.8615   $664.86   $664.85   $664.80