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 $180.48   4.724%   $189.0059   $189.01   $189.00   $189.00  
  Purchase #2 $127.91   8.521%   $138.8092   $138.81   $138.80   $138.80  
  Purchase #3 $75.31   11.301%   $83.8208   $83.82   $83.80   $83.80  
  Purchase #4 $102.51   5.364%   $108.0086   $108.01   $108.00   $108.00  
  Purchase #5 $27.39   9.428%   $29.9723   $29.97   $29.95   $30.00  
  Purchase #6 $230.36   3.670%   $238.8142   $238.81   $238.80   $238.80  
  Purchase #7 $111.64   9.437%   $122.1755   $122.18   $122.20   $122.20  
  7 purchase totals = $855.60       $910.6065   $910.61   $910.55   $910.60