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 $30.40   6.288%   $32.3116   $32.31   $32.30   $32.30  
  Purchase #2 $110.70   3.459%   $114.5291   $114.53   $114.55   $114.50  
  Purchase #3 $143.40   10.683%   $158.7194   $158.72   $158.70   $158.70  
  Purchase #4 $225.87   7.635%   $243.1152   $243.12   $243.10   $243.10  
  Purchase #5 $80.76   4.822%   $84.6542   $84.65   $84.65   $84.70  
  Purchase #6 $13.42   9.713%   $14.7235   $14.72   $14.70   $14.70  
  Purchase #7 $239.26   6.276%   $254.276   $254.28   $254.30   $254.30  
  7 purchase totals = $843.81       $902.3290   $902.33   $902.30   $902.30