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 $120.72   11.824%   $134.9939   $134.99   $135.00   $135.00  
  Purchase #2 $173.98   12.868%   $196.3677   $196.37   $196.35   $196.40  
  Purchase #3 $161.69   8.836%   $175.9769   $175.98   $176.00   $176.00  
  Purchase #4 $89.39   13.740%   $101.6722   $101.67   $101.65   $101.70  
  Purchase #5 $73.27   12.302%   $82.2837   $82.28   $82.30   $82.30  
  Purchase #6 $227.48   8.745%   $247.3731   $247.37   $247.35   $247.40  
  Purchase #7 $44.43   7.661%   $47.8338   $47.83   $47.85   $47.80  
  7 purchase totals = $890.96       $986.5013   $986.49   $986.50   $986.60