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 $18.50   7.416%   $19.872   $19.87   $19.85   $19.90  
  Purchase #2 $209.59   10.854%   $232.3389   $232.34   $232.35   $232.30  
  Purchase #3 $11.93   8.169%   $12.9046   $12.90   $12.90   $12.90  
  Purchase #4 $90.29   13.337%   $102.332   $102.33   $102.35   $102.30  
  Purchase #5 $89.80   4.527%   $93.8652   $93.87   $93.85   $93.90  
  Purchase #6 $24.75   4.563%   $25.8793   $25.88   $25.90   $25.90  
  Purchase #7 $40.90   5.694%   $43.2288   $43.23   $43.25   $43.20  
  7 purchase totals = $485.76       $530.4208   $530.42   $530.45   $530.40