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 $143.81   10.454%   $158.8439   $158.84   $158.85   $158.80  
  Purchase #2 $216.55   6.238%   $230.0584   $230.06   $230.05   $230.10  
  Purchase #3 $76.84   9.635%   $84.2435   $84.24   $84.25   $84.20  
  Purchase #4 $24.25   6.686%   $25.8714   $25.87   $25.85   $25.90  
  Purchase #5 $20.72   5.617%   $21.8838   $21.88   $21.90   $21.90  
  Purchase #6 $117.57   7.289%   $126.1397   $126.14   $126.15   $126.10  
  Purchase #7 $20.44   8.308%   $22.1382   $22.14   $22.15   $22.10  
  7 purchase totals = $620.18       $669.1789   $669.17   $669.20   $669.10