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 $73.37   11.426%   $81.7533   $81.75   $81.75   $81.80  
  Purchase #2 $175.44   12.289%   $196.9998   $197.00   $197.00   $197.00  
  Purchase #3 $27.95   7.189%   $29.9593   $29.96   $29.95   $30.00  
  Purchase #4 $177.56   8.319%   $192.3312   $192.33   $192.35   $192.30  
  Purchase #5 $9.33   3.481%   $9.6548   $9.65   $9.65   $9.70  
  Purchase #6 $203.95   5.617%   $215.4059   $215.41   $215.40   $215.40  
  Purchase #7 $246.33   3.320%   $254.5082   $254.51   $254.50   $254.50  
  7 purchase totals = $913.93       $980.6125   $980.61   $980.60   $980.70