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 $244.76   12.261%   $274.77   $274.77   $274.75   $274.80  
  Purchase #2 $224.61   12.616%   $252.9468   $252.95   $252.95   $252.90  
  Purchase #3 $87.90   5.478%   $92.7152   $92.72   $92.70   $92.70  
  Purchase #4 $121.65   12.662%   $137.0533   $137.05   $137.05   $137.10  
  Purchase #5 $110.28   12.564%   $124.1356   $124.14   $124.15   $124.10  
  Purchase #6 $229.87   10.114%   $253.1191   $253.12   $253.10   $253.10  
  Purchase #7 $121.82   8.861%   $132.6145   $132.61   $132.60   $132.60  
  7 purchase totals = $1,140.89       $1,267.3545   $1,267.36   $1,267.30   $1,267.30