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 $15.45   6.736%   $16.4907   $16.49   $16.50   $16.50  
  Purchase #2 $12.98   3.638%   $13.4522   $13.45   $13.45   $13.50  
  Purchase #3 $150.96   8.532%   $163.8399   $163.84   $163.85   $163.80  
  Purchase #4 $138.72   11.718%   $154.9752   $154.98   $155.00   $155.00  
  Purchase #5 $78.35   11.152%   $87.0876   $87.09   $87.10   $87.10  
  Purchase #6 $71.50   3.837%   $74.2435   $74.24   $74.25   $74.20  
  Purchase #7 $115.27   8.108%   $124.6161   $124.62   $124.60   $124.60  
  7 purchase totals = $583.23       $634.7052   $634.71   $634.75   $634.70