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 $8.40   4.628%   $8.7888   $8.79   $8.80   $8.80  
  Purchase #2 $121.38   6.658%   $129.4615   $129.46   $129.45   $129.50  
  Purchase #3 $102.53   5.170%   $107.8308   $107.83   $107.85   $107.80  
  Purchase #4 $203.79   7.589%   $219.2556   $219.26   $219.25   $219.30  
  Purchase #5 $109.75   5.595%   $115.8905   $115.89   $115.90   $115.90  
  Purchase #6 $80.71   11.320%   $89.8464   $89.85   $89.85   $89.80  
  Purchase #7 $154.41   9.316%   $168.7948   $168.79   $168.80   $168.80  
  7 purchase totals = $780.97       $839.8684   $839.87   $839.90   $839.90