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 $29.29   5.735%   $30.9698   $30.97   $30.95   $31.00  
  Purchase #2 $164.41   3.716%   $170.5195   $170.52   $170.50   $170.50  
  Purchase #3 $7.88   9.242%   $8.6083   $8.61   $8.60   $8.60  
  Purchase #4 $148.97   4.549%   $155.7466   $155.75   $155.75   $155.70  
  Purchase #5 $117.68   7.136%   $126.0776   $126.08   $126.10   $126.10  
  Purchase #6 $100.52   10.162%   $110.7348   $110.73   $110.75   $110.70  
  Purchase #7 $82.84   8.271%   $89.6917   $89.69   $89.70   $89.70  
  7 purchase totals = $651.59       $692.3483   $692.35   $692.35   $692.30