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 $11.31   10.552%   $12.5034   $12.50   $12.50   $12.50  
  Purchase #2 $162.65   5.813%   $172.1048   $172.10   $172.10   $172.10  
  Purchase #3 $31.28   11.646%   $34.9229   $34.92   $34.90   $34.90  
  Purchase #4 $196.79   8.699%   $213.9088   $213.91   $213.90   $213.90  
  Purchase #5 $38.36   8.215%   $41.5113   $41.51   $41.50   $41.50  
  Purchase #6 $91.63   9.460%   $100.2982   $100.30   $100.30   $100.30  
  Purchase #7 $115.70   7.282%   $124.1253   $124.13   $124.15   $124.10  
  7 purchase totals = $647.72       $699.3747   $699.37   $699.35   $699.30