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 $15.67   6.711%   $16.7216   $16.72   $16.70   $16.70  
  Purchase #2 $248.69   8.137%   $268.9259   $268.93   $268.95   $268.90  
  Purchase #3 $141.68   9.144%   $154.6352   $154.64   $154.65   $154.60  
  Purchase #4 $215.67   12.730%   $243.1248   $243.12   $243.10   $243.10  
  Purchase #5 $33.36   4.267%   $34.7835   $34.78   $34.80   $34.80  
  Purchase #6 $62.80   12.460%   $70.6249   $70.62   $70.60   $70.60  
  Purchase #7 $176.89   13.714%   $201.1487   $201.15   $201.15   $201.10  
  7 purchase totals = $894.76       $989.9646   $989.96   $989.95   $989.80