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 $80.61   9.621%   $88.3655   $88.37   $88.35   $88.40  
  Purchase #2 $26.52   9.751%   $29.106   $29.11   $29.10   $29.10  
  Purchase #3 $108.83   12.526%   $122.462   $122.46   $122.45   $122.50  
  Purchase #4 $186.57   13.397%   $211.5648   $211.56   $211.55   $211.60  
  Purchase #5 $15.44   13.558%   $17.5334   $17.53   $17.55   $17.50  
  Purchase #6 $184.80   5.524%   $195.0084   $195.01   $195.00   $195.00  
  Purchase #7 $204.85   11.311%   $228.0206   $228.02   $228.00   $228.00  
  7 purchase totals = $807.62       $892.0607   $892.06   $892.00   $892.10