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 $21.64   12.823%   $24.4149   $24.41   $24.40   $24.40  
  Purchase #2 $198.27   12.603%   $223.258   $223.26   $223.25   $223.30  
  Purchase #3 $135.70   11.356%   $151.1101   $151.11   $151.10   $151.10  
  Purchase #4 $30.45   4.352%   $31.7752   $31.78   $31.80   $31.80  
  Purchase #5 $9.50   7.770%   $10.2382   $10.24   $10.25   $10.20  
  Purchase #6 $8.79   13.658%   $9.9905   $9.99   $10.00   $10.00  
  Purchase #7 $78.25   13.453%   $88.777   $88.78   $88.80   $88.80  
  7 purchase totals = $482.60       $539.5639   $539.57   $539.60   $539.60