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 $61.47   7.374%   $66.0028   $66.00   $66.00   $66.00  
  Purchase #2 $67.25   3.851%   $69.8398   $69.84   $69.85   $69.80  
  Purchase #3 $241.35   13.302%   $273.4544   $273.45   $273.45   $273.50  
  Purchase #4 $92.61   9.590%   $101.4913   $101.49   $101.50   $101.50  
  Purchase #5 $81.67   7.615%   $87.8892   $87.89   $87.90   $87.90  
  Purchase #6 $139.80   4.311%   $145.8268   $145.83   $145.85   $145.80  
  Purchase #7 $26.33   10.410%   $29.071   $29.07   $29.05   $29.10  
  7 purchase totals = $710.48       $773.5753   $773.57   $773.60   $773.60