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 $155.30   3.311%   $160.442   $160.44   $160.45   $160.40  
  Purchase #2 $55.32   12.429%   $62.1957   $62.20   $62.20   $62.20  
  Purchase #3 $177.35   10.743%   $196.4027   $196.40   $196.40   $196.40  
  Purchase #4 $183.81   10.830%   $203.7166   $203.72   $203.70   $203.70  
  Purchase #5 $187.50   13.754%   $213.2888   $213.29   $213.30   $213.30  
  Purchase #6 $113.88   7.253%   $122.1397   $122.14   $122.15   $122.10  
  Purchase #7 $27.73   12.464%   $31.1863   $31.19   $31.20   $31.20  
  7 purchase totals = $900.89       $989.3718   $989.38   $989.40   $989.30