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 $27.29   4.235%   $28.4457   $28.45   $28.45   $28.40  
  Purchase #2 $93.53   10.468%   $103.3207   $103.32   $103.30   $103.30  
  Purchase #3 $26.52   13.835%   $30.189   $30.19   $30.20   $30.20  
  Purchase #4 $174.89   7.372%   $187.7829   $187.78   $187.80   $187.80  
  Purchase #5 $41.41   4.253%   $43.1712   $43.17   $43.15   $43.20  
  Purchase #6 $42.94   8.435%   $46.562   $46.56   $46.55   $46.60  
  Purchase #7 $180.98   9.713%   $198.5586   $198.56   $198.55   $198.60  
  7 purchase totals = $587.56       $638.0301   $638.03   $638.00   $638.10