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 $19.93   4.118%   $20.7507   $20.75   $20.75   $20.80  
  Purchase #2 $78.71   11.538%   $87.7916   $87.79   $87.80   $87.80  
  Purchase #3 $133.33   11.212%   $148.279   $148.28   $148.30   $148.30  
  Purchase #4 $203.67   6.242%   $216.3831   $216.38   $216.40   $216.40  
  Purchase #5 $80.32   10.421%   $88.6901   $88.69   $88.70   $88.70  
  Purchase #6 $31.93   8.457%   $34.6303   $34.63   $34.65   $34.60  
  Purchase #7 $233.56   7.864%   $251.9272   $251.93   $251.95   $251.90  
  7 purchase totals = $781.45       $848.4520   $848.45   $848.55   $848.50