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 $34.43   13.219%   $38.9813   $38.98   $39.00   $39.00  
  Purchase #2 $184.71   9.287%   $201.864   $201.86   $201.85   $201.90  
  Purchase #3 $230.42   13.789%   $262.1926   $262.19   $262.20   $262.20  
  Purchase #4 $201.87   13.711%   $229.5484   $229.55   $229.55   $229.50  
  Purchase #5 $104.53   4.732%   $109.4764   $109.48   $109.50   $109.50  
  Purchase #6 $49.99   8.532%   $54.2551   $54.26   $54.25   $54.30  
  Purchase #7 $116.42   10.339%   $128.4567   $128.46   $128.45   $128.50  
  7 purchase totals = $922.37       $1,024.7745   $1,024.78   $1,024.80   $1,024.90