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 $87.51   13.852%   $99.6319   $99.63   $99.65   $99.60  
  Purchase #2 $31.73   9.130%   $34.6269   $34.63   $34.65   $34.60  
  Purchase #3 $215.31   9.295%   $235.3231   $235.32   $235.30   $235.30  
  Purchase #4 $79.83   3.770%   $82.8396   $82.84   $82.85   $82.80  
  Purchase #5 $124.38   9.229%   $135.859   $135.86   $135.85   $135.90  
  Purchase #6 $10.91   6.847%   $11.657   $11.66   $11.65   $11.70  
  Purchase #7 $148.84   11.774%   $166.3644   $166.36   $166.35   $166.40  
  7 purchase totals = $698.51       $766.3019   $766.30   $766.30   $766.30