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 $29.99   9.779%   $32.9227   $32.92   $32.90   $32.90  
  Purchase #2 $98.97   9.412%   $108.2851   $108.29   $108.30   $108.30  
  Purchase #3 $140.89   6.738%   $150.3832   $150.38   $150.40   $150.40  
  Purchase #4 $34.96   6.729%   $37.3125   $37.31   $37.30   $37.30  
  Purchase #5 $26.83   8.322%   $29.0628   $29.06   $29.05   $29.10  
  Purchase #6 $118.63   6.282%   $126.0823   $126.08   $126.10   $126.10  
  Purchase #7 $36.41   5.423%   $38.3845   $38.38   $38.40   $38.40  
  7 purchase totals = $486.68       $522.4331   $522.42   $522.45   $522.50