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 $240.77   7.567%   $258.9891   $258.99   $259.00   $259.00  
  Purchase #2 $90.73   3.449%   $93.8593   $93.86   $93.85   $93.90  
  Purchase #3 $173.48   9.841%   $190.5522   $190.55   $190.55   $190.60  
  Purchase #4 $126.62   11.828%   $141.5966   $141.60   $141.60   $141.60  
  Purchase #5 $5.68   5.800%   $6.0094   $6.01   $6.00   $6.00  
  Purchase #6 $24.47   5.190%   $25.74   $25.74   $25.75   $25.70  
  Purchase #7 $35.43   5.586%   $37.4091   $37.41   $37.40   $37.40  
  7 purchase totals = $697.18       $754.1557   $754.16   $754.15   $754.20