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 $12.60   10.258%   $13.8925   $13.89   $13.90   $13.90  
  Purchase #2 $78.54   13.286%   $88.9748   $88.97   $88.95   $89.00  
  Purchase #3 $128.88   7.135%   $138.0756   $138.08   $138.10   $138.10  
  Purchase #4 $132.97   6.353%   $141.4176   $141.42   $141.40   $141.40  
  Purchase #5 $66.52   13.422%   $75.4483   $75.45   $75.45   $75.40  
  Purchase #6 $74.77   4.677%   $78.267   $78.27   $78.25   $78.30  
  Purchase #7 $132.57   13.626%   $150.634   $150.63   $150.65   $150.60  
  7 purchase totals = $626.85       $686.7098   $686.71   $686.70   $686.70