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 $79.80   12.660%   $89.9027   $89.90   $89.90   $89.90  
  Purchase #2 $157.29   13.549%   $178.6012   $178.60   $178.60   $178.60  
  Purchase #3 $156.31   12.391%   $175.6784   $175.68   $175.70   $175.70  
  Purchase #4 $28.89   10.781%   $32.0046   $32.00   $32.00   $32.00  
  Purchase #5 $5.47   13.134%   $6.1884   $6.19   $6.20   $6.20  
  Purchase #6 $108.32   3.546%   $112.161   $112.16   $112.15   $112.20  
  Purchase #7 $90.90   3.698%   $94.2615   $94.26   $94.25   $94.30  
  7 purchase totals = $626.98       $688.7978   $688.79   $688.80   $688.90