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 $172.97   9.716%   $189.7758   $189.78   $189.80   $189.80  
  Purchase #2 $153.37   12.316%   $172.259   $172.26   $172.25   $172.30  
  Purchase #3 $152.80   3.256%   $157.7752   $157.78   $157.80   $157.80  
  Purchase #4 $136.82   8.321%   $148.2048   $148.20   $148.20   $148.20  
  Purchase #5 $5.74   5.839%   $6.0752   $6.08   $6.10   $6.10  
  Purchase #6 $139.49   13.814%   $158.7591   $158.76   $158.75   $158.80  
  Purchase #7 $139.40   12.761%   $157.1888   $157.19   $157.20   $157.20  
  7 purchase totals = $900.59       $990.0379   $990.05   $990.10   $990.20