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 $123.40   10.594%   $136.473   $136.47   $136.45   $136.50  
  Purchase #2 $52.89   10.589%   $58.4905   $58.49   $58.50   $58.50  
  Purchase #3 $238.50   5.864%   $252.4856   $252.49   $252.50   $252.50  
  Purchase #4 $169.32   11.403%   $188.6276   $188.63   $188.65   $188.60  
  Purchase #5 $31.63   3.656%   $32.7864   $32.79   $32.80   $32.80  
  Purchase #6 $138.63   8.867%   $150.9223   $150.92   $150.90   $150.90  
  Purchase #7 $91.55   13.681%   $104.075   $104.08   $104.10   $104.10  
  7 purchase totals = $845.92       $923.8604   $923.87   $923.90   $923.90