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 $55.98   4.698%   $58.6099   $58.61   $58.60   $58.60  
  Purchase #2 $168.66   10.834%   $186.9326   $186.93   $186.95   $186.90  
  Purchase #3 $132.31   4.652%   $138.4651   $138.47   $138.45   $138.50  
  Purchase #4 $129.65   13.454%   $147.0931   $147.09   $147.10   $147.10  
  Purchase #5 $85.49   9.677%   $93.7629   $93.76   $93.75   $93.80  
  Purchase #6 $82.84   13.469%   $93.9977   $94.00   $94.00   $94.00  
  Purchase #7 $108.71   10.322%   $119.931   $119.93   $119.95   $119.90  
  7 purchase totals = $763.64       $838.7923   $838.79   $838.80   $838.80