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 $189.95   12.504%   $213.7013   $213.70   $213.70   $213.70  
  Purchase #2 $242.67   12.756%   $273.625   $273.63   $273.65   $273.60  
  Purchase #3 $156.32   9.712%   $171.5018   $171.50   $171.50   $171.50  
  Purchase #4 $55.91   12.539%   $62.9206   $62.92   $62.90   $62.90  
  Purchase #5 $25.32   6.279%   $26.9098   $26.91   $26.90   $26.90  
  Purchase #6 $217.33   12.267%   $243.9899   $243.99   $244.00   $244.00  
  Purchase #7 $238.76   7.692%   $257.1254   $257.13   $257.15   $257.10  
  7 purchase totals = $1,126.26       $1,249.7738   $1,249.78   $1,249.80   $1,249.70