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 $126.87   9.702%   $139.1789   $139.18   $139.20   $139.20  
  Purchase #2 $8.99   3.403%   $9.2959   $9.30   $9.30   $9.30  
  Purchase #3 $153.38   8.849%   $166.9526   $166.95   $166.95   $167.00  
  Purchase #4 $162.65   13.530%   $184.6565   $184.66   $184.65   $184.70  
  Purchase #5 $80.28   10.203%   $88.471   $88.47   $88.45   $88.50  
  Purchase #6 $113.35   10.240%   $124.957   $124.96   $124.95   $125.00  
  Purchase #7 $105.55   3.558%   $109.3055   $109.31   $109.30   $109.30  
  7 purchase totals = $751.07       $822.8174   $822.83   $822.80   $823.00