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 $89.91   4.239%   $93.7213   $93.72   $93.70   $93.70  
  Purchase #2 $181.53   4.365%   $189.4538   $189.45   $189.45   $189.50  
  Purchase #3 $177.65   6.624%   $189.4175   $189.42   $189.40   $189.40  
  Purchase #4 $10.72   9.719%   $11.7619   $11.76   $11.75   $11.80  
  Purchase #5 $80.91   3.347%   $83.6181   $83.62   $83.60   $83.60  
  Purchase #6 $16.81   7.600%   $18.0876   $18.09   $18.10   $18.10  
  Purchase #7 $83.84   12.319%   $94.1682   $94.17   $94.15   $94.20  
  7 purchase totals = $641.37       $680.2284   $680.23   $680.15   $680.30