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 $64.72   6.397%   $68.8601   $68.86   $68.85   $68.90  
  Purchase #2 $91.85   4.168%   $95.6783   $95.68   $95.70   $95.70  
  Purchase #3 $34.44   4.576%   $36.016   $36.02   $36.00   $36.00  
  Purchase #4 $214.42   7.273%   $230.0148   $230.01   $230.00   $230.00  
  Purchase #5 $93.86   11.671%   $104.8144   $104.81   $104.80   $104.80  
  Purchase #6 $89.61   5.208%   $94.2769   $94.28   $94.30   $94.30  
  Purchase #7 $34.71   5.794%   $36.7211   $36.72   $36.70   $36.70  
  7 purchase totals = $623.61       $666.3816   $666.38   $666.35   $666.40