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 $234.29   6.554%   $249.6454   $249.65   $249.65   $249.60  
  Purchase #2 $27.32   11.766%   $30.5345   $30.53   $30.55   $30.50  
  Purchase #3 $97.65   7.745%   $105.213   $105.21   $105.20   $105.20  
  Purchase #4 $66.38   6.714%   $70.8368   $70.84   $70.85   $70.80  
  Purchase #5 $24.45   3.626%   $25.3366   $25.34   $25.35   $25.30  
  Purchase #6 $174.38   11.820%   $194.9917   $194.99   $195.00   $195.00  
  Purchase #7 $234.30   5.455%   $247.0811   $247.08   $247.10   $247.10  
  7 purchase totals = $858.77       $923.6391   $923.64   $923.70   $923.50