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 $19.32   12.828%   $21.7984   $21.80   $21.80   $21.80  
  Purchase #2 $15.96   11.302%   $17.7638   $17.76   $17.75   $17.80  
  Purchase #3 $100.78   11.772%   $112.6438   $112.64   $112.65   $112.60  
  Purchase #4 $187.50   6.288%   $199.29   $199.29   $199.30   $199.30  
  Purchase #5 $20.91   7.486%   $22.4753   $22.48   $22.50   $22.50  
  Purchase #6 $67.71   12.196%   $75.9679   $75.97   $75.95   $76.00  
  Purchase #7 $42.33   4.830%   $44.3745   $44.37   $44.35   $44.40  
  7 purchase totals = $454.51       $494.3137   $494.31   $494.30   $494.40