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 $136.63   12.453%   $153.6445   $153.64   $153.65   $153.60  
  Purchase #2 $152.71   13.833%   $173.8344   $173.83   $173.85   $173.80  
  Purchase #3 $219.94   13.625%   $249.9068   $249.91   $249.90   $249.90  
  Purchase #4 $179.65   5.615%   $189.7373   $189.74   $189.75   $189.70  
  Purchase #5 $140.63   3.557%   $145.6322   $145.63   $145.65   $145.60  
  Purchase #6 $123.74   4.788%   $129.6647   $129.66   $129.65   $129.70  
  Purchase #7 $176.34   8.577%   $191.4647   $191.46   $191.45   $191.50  
  7 purchase totals = $1,129.64       $1,233.8846   $1,233.87   $1,233.90   $1,233.80