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 $28.80   9.360%   $31.4957   $31.50   $31.50   $31.50  
  Purchase #2 $7.43   13.548%   $8.4366   $8.44   $8.45   $8.40  
  Purchase #3 $37.34   5.490%   $39.39   $39.39   $39.40   $39.40  
  Purchase #4 $168.83   8.471%   $183.1316   $183.13   $183.15   $183.10  
  Purchase #5 $11.64   11.294%   $12.9546   $12.95   $12.95   $13.00  
  Purchase #6 $20.74   13.723%   $23.5862   $23.59   $23.60   $23.60  
  Purchase #7 $113.82   6.386%   $121.0885   $121.09   $121.10   $121.10  
  7 purchase totals = $388.60       $420.0832   $420.09   $420.15   $420.10