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 $73.71   7.160%   $78.9876   $78.99   $79.00   $79.00  
  Purchase #2 $35.44   10.399%   $39.1254   $39.13   $39.15   $39.10  
  Purchase #3 $31.91   12.407%   $35.8691   $35.87   $35.85   $35.90  
  Purchase #4 $158.68   11.460%   $176.8647   $176.86   $176.85   $176.90  
  Purchase #5 $11.67   12.845%   $13.169   $13.17   $13.15   $13.20  
  Purchase #6 $146.72   12.721%   $165.3843   $165.38   $165.40   $165.40  
  Purchase #7 $128.87   5.402%   $135.8316   $135.83   $135.85   $135.80  
  7 purchase totals = $587.00       $645.2317   $645.23   $645.25   $645.30