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 $249.41   6.865%   $266.532   $266.53   $266.55   $266.50  
  Purchase #2 $73.42   10.674%   $81.2569   $81.26   $81.25   $81.30  
  Purchase #3 $121.67   11.444%   $135.5939   $135.59   $135.60   $135.60  
  Purchase #4 $14.77   13.757%   $16.8019   $16.80   $16.80   $16.80  
  Purchase #5 $245.95   11.750%   $274.8491   $274.85   $274.85   $274.80  
  Purchase #6 $7.51   11.263%   $8.3559   $8.36   $8.35   $8.40  
  Purchase #7 $185.99   3.150%   $191.8487   $191.85   $191.85   $191.80  
  7 purchase totals = $898.72       $975.2384   $975.24   $975.25   $975.20