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 $35.87   9.537%   $39.2909   $39.29   $39.30   $39.30  
  Purchase #2 $10.53   6.482%   $11.2126   $11.21   $11.20   $11.20  
  Purchase #3 $185.25   13.302%   $209.892   $209.89   $209.90   $209.90  
  Purchase #4 $7.99   12.543%   $8.9922   $8.99   $9.00   $9.00  
  Purchase #5 $196.38   11.798%   $219.5489   $219.55   $219.55   $219.50  
  Purchase #6 $38.56   7.420%   $41.4212   $41.42   $41.40   $41.40  
  Purchase #7 $128.33   13.148%   $145.2028   $145.20   $145.20   $145.20  
  7 purchase totals = $602.91       $675.5606   $675.55   $675.55   $675.50