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 $80.80   11.378%   $89.9934   $89.99   $90.00   $90.00  
  Purchase #2 $170.39   13.692%   $193.7198   $193.72   $193.70   $193.70  
  Purchase #3 $123.82   4.127%   $128.9301   $128.93   $128.95   $128.90  
  Purchase #4 $111.80   13.861%   $127.2966   $127.30   $127.30   $127.30  
  Purchase #5 $29.89   9.170%   $32.6309   $32.63   $32.65   $32.60  
  Purchase #6 $26.82   8.850%   $29.1936   $29.19   $29.20   $29.20  
  Purchase #7 $229.73   4.440%   $239.93   $239.93   $239.95   $239.90  
  7 purchase totals = $773.25       $841.6944   $841.69   $841.75   $841.60