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 $51.48   13.719%   $58.5425   $58.54   $58.55   $58.50  
  Purchase #2 $117.28   9.405%   $128.3102   $128.31   $128.30   $128.30  
  Purchase #3 $224.97   6.495%   $239.5818   $239.58   $239.60   $239.60  
  Purchase #4 $45.95   8.853%   $50.018   $50.02   $50.00   $50.00  
  Purchase #5 $75.92   11.271%   $84.4769   $84.48   $84.50   $84.50  
  Purchase #6 $243.78   13.300%   $276.2027   $276.20   $276.20   $276.20  
  Purchase #7 $122.33   13.364%   $138.6782   $138.68   $138.70   $138.70  
  7 purchase totals = $881.71       $975.8103   $975.81   $975.85   $975.80