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 $34.70   3.715%   $35.9891   $35.99   $36.00   $36.00  
  Purchase #2 $105.41   3.310%   $108.8991   $108.90   $108.90   $108.90  
  Purchase #3 $135.98   7.239%   $145.8236   $145.82   $145.80   $145.80  
  Purchase #4 $33.68   5.106%   $35.3997   $35.40   $35.40   $35.40  
  Purchase #5 $25.54   11.837%   $28.5632   $28.56   $28.55   $28.60  
  Purchase #6 $25.60   13.436%   $29.0396   $29.04   $29.05   $29.00  
  Purchase #7 $144.31   11.292%   $160.6055   $160.61   $160.60   $160.60  
  7 purchase totals = $505.22       $544.3198   $544.32   $544.30   $544.30