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 $230.33   11.380%   $256.5416   $256.54   $256.55   $256.50  
  Purchase #2 $181.31   8.516%   $196.7504   $196.75   $196.75   $196.80  
  Purchase #3 $65.43   4.654%   $68.4751   $68.48   $68.50   $68.50  
  Purchase #4 $244.90   12.289%   $274.9958   $275.00   $275.00   $275.00  
  Purchase #5 $31.39   7.714%   $33.8114   $33.81   $33.80   $33.80  
  Purchase #6 $187.97   13.864%   $214.0302   $214.03   $214.05   $214.00  
  Purchase #7 $219.36   12.428%   $246.6221   $246.62   $246.60   $246.60  
  7 purchase totals = $1,160.69       $1,291.2266   $1,291.23   $1,291.25   $1,291.20