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 $35.48   12.527%   $39.9246   $39.92   $39.90   $39.90  
  Purchase #2 $235.70   3.718%   $244.4633   $244.46   $244.45   $244.50  
  Purchase #3 $55.69   4.215%   $58.0373   $58.04   $58.05   $58.00  
  Purchase #4 $200.32   4.532%   $209.3985   $209.40   $209.40   $209.40  
  Purchase #5 $232.67   5.472%   $245.4017   $245.40   $245.40   $245.40  
  Purchase #6 $245.43   9.590%   $268.9667   $268.97   $268.95   $269.00  
  Purchase #7 $153.84   8.557%   $167.0041   $167.00   $167.00   $167.00  
  7 purchase totals = $1,159.13       $1,233.1962   $1,233.19   $1,233.15   $1,233.20