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 $116.88   11.614%   $130.4544   $130.45   $130.45   $130.50  
  Purchase #2 $235.77   9.331%   $257.7697   $257.77   $257.75   $257.80  
  Purchase #3 $46.50   4.792%   $48.7283   $48.73   $48.75   $48.70  
  Purchase #4 $55.96   6.478%   $59.5851   $59.59   $59.60   $59.60  
  Purchase #5 $94.70   6.772%   $101.1131   $101.11   $101.10   $101.10  
  Purchase #6 $143.94   6.421%   $153.1824   $153.18   $153.20   $153.20  
  Purchase #7 $74.50   6.192%   $79.113   $79.11   $79.10   $79.10  
  7 purchase totals = $768.25       $829.9460   $829.94   $829.95   $830.00