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 $18.50   12.389%   $20.792   $20.79   $20.80   $20.80  
  Purchase #2 $8.26   11.349%   $9.1974   $9.20   $9.20   $9.20  
  Purchase #3 $155.66   11.508%   $173.5734   $173.57   $173.55   $173.60  
  Purchase #4 $135.62   6.212%   $144.0447   $144.04   $144.05   $144.00  
  Purchase #5 $20.87   6.221%   $22.1683   $22.17   $22.15   $22.20  
  Purchase #6 $184.34   3.779%   $191.3062   $191.31   $191.30   $191.30  
  Purchase #7 $179.75   5.530%   $189.6902   $189.69   $189.70   $189.70  
  7 purchase totals = $703.00       $750.7722   $750.77   $750.75   $750.80