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 $13.88   13.829%   $15.7995   $15.80   $15.80   $15.80  
  Purchase #2 $78.98   6.463%   $84.0845   $84.08   $84.10   $84.10  
  Purchase #3 $7.38   6.488%   $7.8588   $7.86   $7.85   $7.90  
  Purchase #4 $185.53   4.272%   $193.4558   $193.46   $193.45   $193.50  
  Purchase #5 $134.41   6.450%   $143.0794   $143.08   $143.10   $143.10  
  Purchase #6 $26.32   13.692%   $29.9237   $29.92   $29.90   $29.90  
  Purchase #7 $160.74   4.296%   $167.6454   $167.65   $167.65   $167.60  
  7 purchase totals = $607.24       $641.8471   $641.85   $641.85   $641.90