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 $62.97   11.656%   $70.3098   $70.31   $70.30   $70.30  
  Purchase #2 $138.83   11.539%   $154.8496   $154.85   $154.85   $154.80  
  Purchase #3 $202.85   7.315%   $217.6885   $217.69   $217.70   $217.70  
  Purchase #4 $200.54   8.103%   $216.7898   $216.79   $216.80   $216.80  
  Purchase #5 $120.66   4.688%   $126.3165   $126.32   $126.30   $126.30  
  Purchase #6 $11.99   4.862%   $12.573   $12.57   $12.55   $12.60  
  Purchase #7 $9.51   7.450%   $10.2185   $10.22   $10.20   $10.20  
  7 purchase totals = $747.35       $808.7457   $808.75   $808.70   $808.70