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 $17.56   7.232%   $18.8299   $18.83   $18.85   $18.80  
  Purchase #2 $157.75   12.768%   $177.8915   $177.89   $177.90   $177.90  
  Purchase #3 $9.43   4.780%   $9.8808   $9.88   $9.90   $9.90  
  Purchase #4 $30.30   8.433%   $32.8552   $32.86   $32.85   $32.90  
  Purchase #5 $192.84   8.809%   $209.8273   $209.83   $209.85   $209.80  
  Purchase #6 $63.64   3.372%   $65.7859   $65.79   $65.80   $65.80  
  Purchase #7 $65.27   9.290%   $71.3336   $71.33   $71.35   $71.30  
  7 purchase totals = $536.79       $586.4042   $586.41   $586.50   $586.40