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 $119.85   13.529%   $136.0645   $136.06   $136.05   $136.10  
  Purchase #2 $220.29   9.698%   $241.6537   $241.65   $241.65   $241.70  
  Purchase #3 $15.90   13.781%   $18.0912   $18.09   $18.10   $18.10  
  Purchase #4 $59.71   8.778%   $64.9513   $64.95   $64.95   $65.00  
  Purchase #5 $249.94   12.572%   $281.3625   $281.36   $281.35   $281.40  
  Purchase #6 $52.55   9.203%   $57.3862   $57.39   $57.40   $57.40  
  Purchase #7 $155.83   12.108%   $174.6979   $174.70   $174.70   $174.70  
  7 purchase totals = $874.07       $974.2073   $974.20   $974.20   $974.40