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 $29.76   6.550%   $31.7093   $31.71   $31.70   $31.70  
  Purchase #2 $70.45   12.828%   $79.4873   $79.49   $79.50   $79.50  
  Purchase #3 $136.32   10.164%   $150.1756   $150.18   $150.20   $150.20  
  Purchase #4 $207.63   13.469%   $235.5957   $235.60   $235.60   $235.60  
  Purchase #5 $96.36   13.413%   $109.2848   $109.28   $109.30   $109.30  
  Purchase #6 $98.44   12.685%   $110.9271   $110.93   $110.95   $110.90  
  Purchase #7 $29.51   5.703%   $31.193   $31.19   $31.20   $31.20  
  7 purchase totals = $668.47       $748.3728   $748.38   $748.45   $748.40