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 $25.54   10.324%   $28.1767   $28.18   $28.20   $28.20  
  Purchase #2 $133.61   10.681%   $147.8809   $147.88   $147.90   $147.90  
  Purchase #3 $16.57   13.735%   $18.8459   $18.85   $18.85   $18.80  
  Purchase #4 $129.87   3.200%   $134.0258   $134.03   $134.05   $134.00  
  Purchase #5 $111.99   5.186%   $117.7978   $117.80   $117.80   $117.80  
  Purchase #6 $79.73   12.831%   $89.9602   $89.96   $89.95   $90.00  
  Purchase #7 $158.74   10.108%   $174.7854   $174.79   $174.80   $174.80  
  7 purchase totals = $656.05       $711.4727   $711.49   $711.55   $711.50