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 $17.78   12.441%   $19.992   $19.99   $20.00   $20.00  
  Purchase #2 $210.53   4.154%   $219.2754   $219.28   $219.30   $219.30  
  Purchase #3 $216.53   12.352%   $243.2758   $243.28   $243.30   $243.30  
  Purchase #4 $85.79   5.791%   $90.7581   $90.76   $90.75   $90.80  
  Purchase #5 $89.70   10.215%   $98.8629   $98.86   $98.85   $98.90  
  Purchase #6 $11.85   8.597%   $12.8687   $12.87   $12.85   $12.90  
  Purchase #7 $115.94   3.767%   $120.3075   $120.31   $120.30   $120.30  
  7 purchase totals = $748.12       $805.3404   $805.35   $805.35   $805.50