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 $112.82   10.440%   $124.5984   $124.60   $124.60   $124.60  
  Purchase #2 $87.84   11.124%   $97.6113   $97.61   $97.60   $97.60  
  Purchase #3 $153.49   10.342%   $169.3639   $169.36   $169.35   $169.40  
  Purchase #4 $134.26   13.504%   $152.3905   $152.39   $152.40   $152.40  
  Purchase #5 $48.56   12.436%   $54.5989   $54.60   $54.60   $54.60  
  Purchase #6 $163.84   3.280%   $169.214   $169.21   $169.20   $169.20  
  Purchase #7 $11.41   5.377%   $12.0235   $12.02   $12.00   $12.00  
  7 purchase totals = $712.22       $779.8005   $779.79   $779.75   $779.80