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 $28.99   3.732%   $30.0719   $30.07   $30.05   $30.10  
  Purchase #2 $95.37   4.132%   $99.3107   $99.31   $99.30   $99.30  
  Purchase #3 $132.69   11.289%   $147.6694   $147.67   $147.65   $147.70  
  Purchase #4 $107.48   13.313%   $121.7888   $121.79   $121.80   $121.80  
  Purchase #5 $16.51   13.372%   $18.7177   $18.72   $18.70   $18.70  
  Purchase #6 $59.54   5.303%   $62.6974   $62.70   $62.70   $62.70  
  Purchase #7 $97.90   13.490%   $111.1067   $111.11   $111.10   $111.10  
  7 purchase totals = $538.48       $591.3626   $591.37   $591.30   $591.40