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 $25.74   12.365%   $28.9228   $28.92   $28.90   $28.90  
  Purchase #2 $132.40   10.187%   $145.8876   $145.89   $145.90   $145.90  
  Purchase #3 $27.91   12.258%   $31.3312   $31.33   $31.35   $31.30  
  Purchase #4 $148.84   11.536%   $166.0102   $166.01   $166.00   $166.00  
  Purchase #5 $149.99   9.262%   $163.8821   $163.88   $163.90   $163.90  
  Purchase #6 $155.87   8.302%   $168.8103   $168.81   $168.80   $168.80  
  Purchase #7 $243.47   9.748%   $267.2035   $267.20   $267.20   $267.20  
  7 purchase totals = $884.22       $972.0477   $972.04   $972.05   $972.00