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 $235.70   13.273%   $266.9845   $266.98   $267.00   $267.00  
  Purchase #2 $180.49   7.563%   $194.1405   $194.14   $194.15   $194.10  
  Purchase #3 $147.90   9.274%   $161.6162   $161.62   $161.60   $161.60  
  Purchase #4 $18.63   9.296%   $20.3618   $20.36   $20.35   $20.40  
  Purchase #5 $233.35   7.575%   $251.0263   $251.03   $251.05   $251.00  
  Purchase #6 $39.49   11.394%   $43.9895   $43.99   $44.00   $44.00  
  Purchase #7 $242.98   3.588%   $251.6981   $251.70   $251.70   $251.70  
  7 purchase totals = $1,098.54       $1,189.8169   $1,189.82   $1,189.85   $1,189.80