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 $5.93   7.862%   $6.3962   $6.40   $6.40   $6.40  
  Purchase #2 $28.73   9.440%   $31.4421   $31.44   $31.45   $31.40  
  Purchase #3 $54.30   11.352%   $60.4641   $60.46   $60.45   $60.50  
  Purchase #4 $248.36   12.784%   $280.1103   $280.11   $280.10   $280.10  
  Purchase #5 $135.26   13.455%   $153.4592   $153.46   $153.45   $153.50  
  Purchase #6 $101.80   10.496%   $112.4849   $112.48   $112.50   $112.50  
  Purchase #7 $115.57   3.704%   $119.8507   $119.85   $119.85   $119.90  
  7 purchase totals = $689.95       $764.2075   $764.20   $764.20   $764.30