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 $11.71   11.384%   $13.0431   $13.04   $13.05   $13.00  
  Purchase #2 $12.35   12.639%   $13.9109   $13.91   $13.90   $13.90  
  Purchase #3 $248.27   12.466%   $279.2193   $279.22   $279.20   $279.20  
  Purchase #4 $64.36   10.800%   $71.3109   $71.31   $71.30   $71.30  
  Purchase #5 $140.40   4.838%   $147.1926   $147.19   $147.20   $147.20  
  Purchase #6 $237.71   4.550%   $248.5258   $248.53   $248.55   $248.50  
  Purchase #7 $108.68   3.648%   $112.6446   $112.64   $112.65   $112.60  
  7 purchase totals = $823.48       $885.8472   $885.84   $885.85   $885.70