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 $48.64   13.154%   $55.0381   $55.04   $55.05   $55.00  
  Purchase #2 $240.57   5.566%   $253.9601   $253.96   $253.95   $254.00  
  Purchase #3 $128.41   7.523%   $138.0703   $138.07   $138.05   $138.10  
  Purchase #4 $92.27   3.774%   $95.7523   $95.75   $95.75   $95.80  
  Purchase #5 $10.55   13.288%   $11.9519   $11.95   $11.95   $12.00  
  Purchase #6 $136.98   6.406%   $145.7549   $145.75   $145.75   $145.80  
  Purchase #7 $115.53   6.130%   $122.612   $122.61   $122.60   $122.60  
  7 purchase totals = $772.95       $823.1396   $823.13   $823.10   $823.30