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 $102.45   12.413%   $115.1671   $115.17   $115.15   $115.20  
  Purchase #2 $34.62   9.413%   $37.8788   $37.88   $37.90   $37.90  
  Purchase #3 $82.40   6.475%   $87.7354   $87.74   $87.75   $87.70  
  Purchase #4 $176.51   7.529%   $189.7994   $189.80   $189.80   $189.80  
  Purchase #5 $157.62   8.437%   $170.9184   $170.92   $170.90   $170.90  
  Purchase #6 $110.48   4.636%   $115.6019   $115.60   $115.60   $115.60  
  Purchase #7 $151.64   13.731%   $172.4617   $172.46   $172.45   $172.50  
  7 purchase totals = $815.72       $889.5627   $889.57   $889.55   $889.60