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 $60.69   6.196%   $64.4504   $64.45   $64.45   $64.50  
  Purchase #2 $20.49   11.145%   $22.7736   $22.77   $22.75   $22.80  
  Purchase #3 $69.30   5.469%   $73.09   $73.09   $73.10   $73.10  
  Purchase #4 $93.68   3.372%   $96.8389   $96.84   $96.85   $96.80  
  Purchase #5 $114.71   10.415%   $126.657   $126.66   $126.65   $126.70  
  Purchase #6 $232.67   10.463%   $257.0143   $257.01   $257.00   $257.00  
  Purchase #7 $132.92   3.205%   $137.1801   $137.18   $137.20   $137.20  
  7 purchase totals = $724.46       $778.0043   $778.00   $778.00   $778.10