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 $59.52   12.545%   $66.9868   $66.99   $67.00   $67.00  
  Purchase #2 $83.41   7.200%   $89.4155   $89.42   $89.40   $89.40  
  Purchase #3 $124.56   11.162%   $138.4634   $138.46   $138.45   $138.50  
  Purchase #4 $202.96   7.225%   $217.6239   $217.62   $217.60   $217.60  
  Purchase #5 $59.91   13.492%   $67.9931   $67.99   $68.00   $68.00  
  Purchase #6 $160.75   9.792%   $176.4906   $176.49   $176.50   $176.50  
  Purchase #7 $72.54   3.436%   $75.0325   $75.03   $75.05   $75.00  
  7 purchase totals = $763.65       $832.0058   $832.00   $832.00   $832.00