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 $44.39   13.115%   $50.2117   $50.21   $50.20   $50.20  
  Purchase #2 $165.55   4.324%   $172.7084   $172.71   $172.70   $172.70  
  Purchase #3 $195.29   13.158%   $220.9863   $220.99   $221.00   $221.00  
  Purchase #4 $26.29   10.582%   $29.072   $29.07   $29.05   $29.10  
  Purchase #5 $99.61   13.874%   $113.4299   $113.43   $113.45   $113.40  
  Purchase #6 $34.62   6.346%   $36.817   $36.82   $36.80   $36.80  
  Purchase #7 $209.55   10.851%   $232.2883   $232.29   $232.30   $232.30  
  7 purchase totals = $775.30       $855.5136   $855.52   $855.50   $855.50