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 $79.75   4.554%   $83.3818   $83.38   $83.40   $83.40  
  Purchase #2 $51.40   7.118%   $55.0587   $55.06   $55.05   $55.10  
  Purchase #3 $28.97   6.495%   $30.8516   $30.85   $30.85   $30.90  
  Purchase #4 $170.89   10.172%   $188.2729   $188.27   $188.25   $188.30  
  Purchase #5 $207.55   12.359%   $233.2011   $233.20   $233.20   $233.20  
  Purchase #6 $63.64   10.778%   $70.4991   $70.50   $70.50   $70.50  
  Purchase #7 $243.80   3.142%   $251.4602   $251.46   $251.45   $251.50  
  7 purchase totals = $846.00       $912.7254   $912.72   $912.70   $912.90