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 $65.31   6.618%   $69.6322   $69.63   $69.65   $69.60  
  Purchase #2 $11.49   9.582%   $12.591   $12.59   $12.60   $12.60  
  Purchase #3 $75.35   11.497%   $84.013   $84.01   $84.00   $84.00  
  Purchase #4 $56.32   5.416%   $59.3703   $59.37   $59.35   $59.40  
  Purchase #5 $46.82   4.578%   $48.9634   $48.96   $48.95   $49.00  
  Purchase #6 $91.30   5.500%   $96.3215   $96.32   $96.30   $96.30  
  Purchase #7 $166.87   11.577%   $186.1885   $186.19   $186.20   $186.20  
  7 purchase totals = $513.46       $557.0799   $557.07   $557.05   $557.10