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 $5.49   6.423%   $5.8426   $5.84   $5.85   $5.80  
  Purchase #2 $133.66   6.589%   $142.4669   $142.47   $142.45   $142.50  
  Purchase #3 $191.67   8.731%   $208.4047   $208.40   $208.40   $208.40  
  Purchase #4 $28.50   8.786%   $31.004   $31.00   $31.00   $31.00  
  Purchase #5 $33.33   5.138%   $35.0425   $35.04   $35.05   $35.00  
  Purchase #6 $173.78   8.208%   $188.0439   $188.04   $188.05   $188.00  
  Purchase #7 $23.89   6.127%   $25.3537   $25.35   $25.35   $25.40  
  7 purchase totals = $590.32       $636.1583   $636.14   $636.15   $636.10