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 $82.33   9.509%   $90.1588   $90.16   $90.15   $90.20  
  Purchase #2 $7.43   7.268%   $7.97   $7.97   $7.95   $8.00  
  Purchase #3 $124.47   12.561%   $140.1047   $140.10   $140.10   $140.10  
  Purchase #4 $205.52   7.477%   $220.8867   $220.89   $220.90   $220.90  
  Purchase #5 $29.85   3.206%   $30.807   $30.81   $30.80   $30.80  
  Purchase #6 $217.93   5.470%   $229.8508   $229.85   $229.85   $229.90  
  Purchase #7 $16.88   11.452%   $18.8131   $18.81   $18.80   $18.80  
  7 purchase totals = $684.41       $738.5911   $738.59   $738.55   $738.70