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 $78.27   3.650%   $81.1269   $81.13   $81.15   $81.10  
  Purchase #2 $5.66   13.596%   $6.4295   $6.43   $6.45   $6.40  
  Purchase #3 $52.29   7.474%   $56.1982   $56.20   $56.20   $56.20  
  Purchase #4 $205.64   7.736%   $221.5483   $221.55   $221.55   $221.50  
  Purchase #5 $237.40   7.435%   $255.0507   $255.05   $255.05   $255.10  
  Purchase #6 $54.63   8.346%   $59.1894   $59.19   $59.20   $59.20  
  Purchase #7 $35.44   8.130%   $38.3213   $38.32   $38.30   $38.30  
  7 purchase totals = $669.33       $717.8643   $717.87   $717.90   $717.80