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 $73.47   7.708%   $79.1331   $79.13   $79.15   $79.10  
  Purchase #2 $91.42   7.108%   $97.9181   $97.92   $97.90   $97.90  
  Purchase #3 $68.52   3.396%   $70.8469   $70.85   $70.85   $70.80  
  Purchase #4 $32.40   8.294%   $35.0873   $35.09   $35.10   $35.10  
  Purchase #5 $25.88   12.732%   $29.175   $29.18   $29.20   $29.20  
  Purchase #6 $149.70   3.391%   $154.7763   $154.78   $154.80   $154.80  
  Purchase #7 $67.47   4.709%   $70.6472   $70.65   $70.65   $70.60  
  7 purchase totals = $508.86       $537.5839   $537.60   $537.65   $537.50