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 $224.44   12.874%   $253.3344   $253.33   $253.35   $253.30  
  Purchase #2 $5.42   13.689%   $6.1619   $6.16   $6.15   $6.20  
  Purchase #3 $129.69   5.607%   $136.9617   $136.96   $136.95   $137.00  
  Purchase #4 $87.76   10.274%   $96.7765   $96.78   $96.80   $96.80  
  Purchase #5 $109.53   5.551%   $115.61   $115.61   $115.60   $115.60  
  Purchase #6 $156.32   10.673%   $173.004   $173.00   $173.00   $173.00  
  Purchase #7 $195.83   13.288%   $221.8519   $221.85   $221.85   $221.90  
  7 purchase totals = $908.99       $1,003.7004   $1,003.69   $1,003.70   $1,003.80