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 $244.49   13.520%   $277.545   $277.55   $277.55   $277.50  
  Purchase #2 $87.30   8.156%   $94.4202   $94.42   $94.40   $94.40  
  Purchase #3 $199.44   13.113%   $225.5926   $225.59   $225.60   $225.60  
  Purchase #4 $28.84   11.869%   $32.263   $32.26   $32.25   $32.30  
  Purchase #5 $36.39   10.610%   $40.251   $40.25   $40.25   $40.30  
  Purchase #6 $201.92   9.426%   $220.953   $220.95   $220.95   $221.00  
  Purchase #7 $101.75   4.232%   $106.0561   $106.06   $106.05   $106.10  
  7 purchase totals = $900.13       $997.0809   $997.08   $997.05   $997.20