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 $6.96   12.165%   $7.8067   $7.81   $7.80   $7.80  
  Purchase #2 $216.79   6.770%   $231.4667   $231.47   $231.45   $231.50  
  Purchase #3 $107.56   5.229%   $113.1843   $113.18   $113.20   $113.20  
  Purchase #4 $154.56   10.690%   $171.0825   $171.08   $171.10   $171.10  
  Purchase #5 $26.54   5.727%   $28.0599   $28.06   $28.05   $28.10  
  Purchase #6 $40.65   3.219%   $41.9585   $41.96   $41.95   $42.00  
  Purchase #7 $38.33   10.657%   $42.4148   $42.41   $42.40   $42.40  
  7 purchase totals = $591.39       $635.9734   $635.97   $635.95   $636.10