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 $27.40   11.473%   $30.5436   $30.54   $30.55   $30.50  
  Purchase #2 $180.52   4.706%   $189.0153   $189.02   $189.00   $189.00  
  Purchase #3 $132.73   7.385%   $142.5321   $142.53   $142.55   $142.50  
  Purchase #4 $222.66   3.801%   $231.1233   $231.12   $231.10   $231.10  
  Purchase #5 $220.59   6.570%   $235.0828   $235.08   $235.10   $235.10  
  Purchase #6 $178.82   7.235%   $191.7576   $191.76   $191.75   $191.80  
  Purchase #7 $160.75   11.416%   $179.1012   $179.10   $179.10   $179.10  
  7 purchase totals = $1,123.47       $1,199.1559   $1,199.15   $1,199.15   $1,199.10