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 $223.52   12.183%   $250.7514   $250.75   $250.75   $250.80  
  Purchase #2 $243.36   11.163%   $270.5263   $270.53   $270.55   $270.50  
  Purchase #3 $64.59   8.439%   $70.0408   $70.04   $70.05   $70.00  
  Purchase #4 $20.96   10.203%   $23.0985   $23.10   $23.10   $23.10  
  Purchase #5 $109.95   4.230%   $114.6009   $114.60   $114.60   $114.60  
  Purchase #6 $58.45   8.212%   $63.2499   $63.25   $63.25   $63.20  
  Purchase #7 $174.43   6.651%   $186.0313   $186.03   $186.05   $186.00  
  7 purchase totals = $895.26       $978.2991   $978.30   $978.35   $978.20