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 $38.67   3.144%   $39.8858   $39.89   $39.90   $39.90  
  Purchase #2 $12.51   8.813%   $13.6125   $13.61   $13.60   $13.60  
  Purchase #3 $88.33   3.817%   $91.7016   $91.70   $91.70   $91.70  
  Purchase #4 $8.68   12.810%   $9.7919   $9.79   $9.80   $9.80  
  Purchase #5 $111.63   12.751%   $125.8639   $125.86   $125.85   $125.90  
  Purchase #6 $134.60   4.529%   $140.696   $140.70   $140.70   $140.70  
  Purchase #7 $164.90   11.637%   $184.0894   $184.09   $184.10   $184.10  
  7 purchase totals = $559.32       $605.6411   $605.64   $605.65   $605.70