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 $121.52   7.127%   $130.1807   $130.18   $130.20   $130.20  
  Purchase #2 $138.64   5.722%   $146.573   $146.57   $146.55   $146.60  
  Purchase #3 $8.60   10.481%   $9.5014   $9.50   $9.50   $9.50  
  Purchase #4 $25.60   5.134%   $26.9143   $26.91   $26.90   $26.90  
  Purchase #5 $110.34   11.283%   $122.7897   $122.79   $122.80   $122.80  
  Purchase #6 $51.30   6.641%   $54.7068   $54.71   $54.70   $54.70  
  Purchase #7 $29.78   6.655%   $31.7619   $31.76   $31.75   $31.80  
  7 purchase totals = $485.78       $522.4278   $522.42   $522.40   $522.50