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 $107.78   9.212%   $117.7087   $117.71   $117.70   $117.70  
  Purchase #2 $104.99   5.780%   $111.0584   $111.06   $111.05   $111.10  
  Purchase #3 $39.85   12.207%   $44.7145   $44.71   $44.70   $44.70  
  Purchase #4 $96.54   6.212%   $102.5371   $102.54   $102.55   $102.50  
  Purchase #5 $10.50   6.654%   $11.1987   $11.20   $11.20   $11.20  
  Purchase #6 $45.35   3.610%   $46.9871   $46.99   $47.00   $47.00  
  Purchase #7 $82.84   8.608%   $89.9709   $89.97   $89.95   $90.00  
  7 purchase totals = $487.85       $524.1754   $524.18   $524.15   $524.20