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 $20.32   3.288%   $20.9881   $20.99   $21.00   $21.00  
  Purchase #2 $163.60   10.536%   $180.8369   $180.84   $180.85   $180.80  
  Purchase #3 $72.73   10.402%   $80.2954   $80.30   $80.30   $80.30  
  Purchase #4 $210.95   3.812%   $218.9914   $218.99   $219.00   $219.00  
  Purchase #5 $246.74   9.352%   $269.8151   $269.82   $269.80   $269.80  
  Purchase #6 $72.55   5.154%   $76.2892   $76.29   $76.30   $76.30  
  Purchase #7 $174.80   9.407%   $191.2434   $191.24   $191.25   $191.20  
  7 purchase totals = $961.69       $1,038.4595   $1,038.47   $1,038.50   $1,038.40