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 $20.30   6.601%   $21.64   $21.64   $21.65   $21.60  
  Purchase #2 $170.60   4.488%   $178.2565   $178.26   $178.25   $178.30  
  Purchase #3 $5.83   7.145%   $6.2466   $6.25   $6.25   $6.20  
  Purchase #4 $100.44   9.499%   $109.9808   $109.98   $110.00   $110.00  
  Purchase #5 $184.33   8.500%   $199.9981   $200.00   $200.00   $200.00  
  Purchase #6 $233.27   10.479%   $257.7144   $257.71   $257.70   $257.70  
  Purchase #7 $39.85   5.401%   $42.0023   $42.00   $42.00   $42.00  
  7 purchase totals = $754.62       $815.8387   $815.84   $815.85   $815.80