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 $228.37   13.834%   $259.9627   $259.96   $259.95   $260.00  
  Purchase #2 $134.57   10.245%   $148.3567   $148.36   $148.35   $148.40  
  Purchase #3 $83.96   9.125%   $91.6214   $91.62   $91.60   $91.60  
  Purchase #4 $230.26   8.476%   $249.7768   $249.78   $249.80   $249.80  
  Purchase #5 $87.44   7.586%   $94.0732   $94.07   $94.05   $94.10  
  Purchase #6 $162.31   6.849%   $173.4266   $173.43   $173.45   $173.40  
  Purchase #7 $26.94   11.628%   $30.0726   $30.07   $30.05   $30.10  
  7 purchase totals = $953.85       $1,047.2900   $1,047.29   $1,047.25   $1,047.40