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 $244.62   5.453%   $257.9591   $257.96   $257.95   $258.00  
  Purchase #2 $65.37   8.782%   $71.1108   $71.11   $71.10   $71.10  
  Purchase #3 $69.97   9.244%   $76.438   $76.44   $76.45   $76.40  
  Purchase #4 $20.37   13.470%   $23.1138   $23.11   $23.10   $23.10  
  Purchase #5 $94.83   9.381%   $103.726   $103.73   $103.75   $103.70  
  Purchase #6 $201.94   12.572%   $227.3279   $227.33   $227.35   $227.30  
  Purchase #7 $29.89   6.247%   $31.7572   $31.76   $31.75   $31.80  
  7 purchase totals = $726.99       $791.4328   $791.44   $791.45   $791.40