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 $155.90   10.827%   $172.7793   $172.78   $172.80   $172.80  
  Purchase #2 $76.73   6.534%   $81.7435   $81.74   $81.75   $81.70  
  Purchase #3 $93.61   3.873%   $97.2355   $97.24   $97.25   $97.20  
  Purchase #4 $132.64   13.524%   $150.5782   $150.58   $150.60   $150.60  
  Purchase #5 $222.46   4.512%   $232.4974   $232.50   $232.50   $232.50  
  Purchase #6 $69.51   9.117%   $75.8472   $75.85   $75.85   $75.80  
  Purchase #7 $24.39   7.640%   $26.2534   $26.25   $26.25   $26.30  
  7 purchase totals = $775.24       $836.9345   $836.94   $837.00   $836.90