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 $15.91   11.670%   $17.7667   $17.77   $17.75   $17.80  
  Purchase #2 $196.26   12.173%   $220.1507   $220.15   $220.15   $220.20  
  Purchase #3 $37.55   5.259%   $39.5248   $39.52   $39.50   $39.50  
  Purchase #4 $19.63   7.140%   $21.0316   $21.03   $21.05   $21.00  
  Purchase #5 $69.28   6.220%   $73.5892   $73.59   $73.60   $73.60  
  Purchase #6 $203.58   13.825%   $231.7249   $231.72   $231.70   $231.70  
  Purchase #7 $15.41   3.295%   $15.9178   $15.92   $15.90   $15.90  
  7 purchase totals = $557.62       $619.7057   $619.70   $619.65   $619.70