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 $57.63   10.754%   $63.8275   $63.83   $63.85   $63.80  
  Purchase #2 $199.29   9.672%   $218.5653   $218.57   $218.55   $218.60  
  Purchase #3 $175.68   5.516%   $185.3705   $185.37   $185.35   $185.40  
  Purchase #4 $25.84   5.553%   $27.2749   $27.27   $27.25   $27.30  
  Purchase #5 $41.94   6.858%   $44.8162   $44.82   $44.80   $44.80  
  Purchase #6 $26.98   7.370%   $28.9684   $28.97   $28.95   $29.00  
  Purchase #7 $22.92   5.740%   $24.2356   $24.24   $24.25   $24.20  
  7 purchase totals = $550.28       $593.0584   $593.07   $593.00   $593.10