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 $171.50   10.728%   $189.8985   $189.90   $189.90   $189.90  
  Purchase #2 $152.89   4.236%   $159.3664   $159.37   $159.35   $159.40  
  Purchase #3 $166.87   13.510%   $189.4141   $189.41   $189.40   $189.40  
  Purchase #4 $181.80   9.259%   $198.6329   $198.63   $198.65   $198.60  
  Purchase #5 $212.40   3.785%   $220.4393   $220.44   $220.45   $220.40  
  Purchase #6 $125.91   8.860%   $137.0656   $137.07   $137.05   $137.10  
  Purchase #7 $144.89   3.276%   $149.6366   $149.64   $149.65   $149.60  
  7 purchase totals = $1,156.26       $1,244.4534   $1,244.46   $1,244.45   $1,244.40