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   8.548%   $20.0814   $20.08   $20.10   $20.10  
  Purchase #2 $199.52   4.594%   $208.6859   $208.69   $208.70   $208.70  
  Purchase #3 $67.26   3.276%   $69.4634   $69.46   $69.45   $69.50  
  Purchase #4 $17.86   8.194%   $19.3234   $19.32   $19.30   $19.30  
  Purchase #5 $115.38   9.604%   $126.4611   $126.46   $126.45   $126.50  
  Purchase #6 $131.86   10.651%   $145.9044   $145.90   $145.90   $145.90  
  Purchase #7 $54.55   11.723%   $60.9449   $60.94   $60.95   $60.90  
  7 purchase totals = $604.93       $650.8645   $650.85   $650.85   $650.90