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 $44.86   12.551%   $50.4904   $50.49   $50.50   $50.50  
  Purchase #2 $112.95   6.235%   $119.9924   $119.99   $120.00   $120.00  
  Purchase #3 $89.58   5.272%   $94.3027   $94.30   $94.30   $94.30  
  Purchase #4 $243.74   9.760%   $267.529   $267.53   $267.55   $267.50  
  Purchase #5 $79.35   10.138%   $87.3945   $87.39   $87.40   $87.40  
  Purchase #6 $97.36   11.322%   $108.3831   $108.38   $108.40   $108.40  
  Purchase #7 $27.90   9.310%   $30.4975   $30.50   $30.50   $30.50  
  7 purchase totals = $695.74       $758.5896   $758.58   $758.65   $758.60