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 $27.51   9.135%   $30.023   $30.02   $30.00   $30.00  
  Purchase #2 $203.95   4.452%   $213.0299   $213.03   $213.05   $213.00  
  Purchase #3 $213.48   9.136%   $232.9835   $232.98   $233.00   $233.00  
  Purchase #4 $241.25   9.800%   $264.8925   $264.89   $264.90   $264.90  
  Purchase #5 $5.67   11.461%   $6.3198   $6.32   $6.30   $6.30  
  Purchase #6 $66.34   12.166%   $74.4109   $74.41   $74.40   $74.40  
  Purchase #7 $112.59   6.746%   $120.1853   $120.19   $120.20   $120.20  
  7 purchase totals = $870.79       $941.8449   $941.84   $941.85   $941.80