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 $50.89   13.660%   $57.8416   $57.84   $57.85   $57.80  
  Purchase #2 $135.26   3.413%   $139.8764   $139.88   $139.90   $139.90  
  Purchase #3 $75.79   11.459%   $84.4748   $84.47   $84.45   $84.50  
  Purchase #4 $19.69   9.366%   $21.5342   $21.53   $21.55   $21.50  
  Purchase #5 $99.62   12.679%   $112.2508   $112.25   $112.25   $112.30  
  Purchase #6 $146.92   7.708%   $158.2446   $158.24   $158.25   $158.20  
  Purchase #7 $7.95   12.536%   $8.9466   $8.95   $8.95   $8.90  
  7 purchase totals = $536.12       $583.1690   $583.16   $583.20   $583.10