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 $225.97   13.654%   $256.8239   $256.82   $256.80   $256.80  
  Purchase #2 $154.53   12.377%   $173.6562   $173.66   $173.65   $173.70  
  Purchase #3 $79.57   6.547%   $84.7794   $84.78   $84.80   $84.80  
  Purchase #4 $201.28   9.529%   $220.46   $220.46   $220.45   $220.50  
  Purchase #5 $185.36   12.703%   $208.9063   $208.91   $208.90   $208.90  
  Purchase #6 $88.48   13.714%   $100.6141   $100.61   $100.60   $100.60  
  Purchase #7 $189.68   12.561%   $213.5057   $213.51   $213.50   $213.50  
  7 purchase totals = $1,124.87       $1,258.7456   $1,258.75   $1,258.70   $1,258.80