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 $173.72   6.156%   $184.4142   $184.41   $184.40   $184.40  
  Purchase #2 $72.36   10.758%   $80.1445   $80.14   $80.15   $80.10  
  Purchase #3 $29.66   8.477%   $32.1743   $32.17   $32.15   $32.20  
  Purchase #4 $130.70   7.355%   $140.313   $140.31   $140.30   $140.30  
  Purchase #5 $170.38   7.420%   $183.0222   $183.02   $183.00   $183.00  
  Purchase #6 $172.92   8.648%   $187.8741   $187.87   $187.85   $187.90  
  Purchase #7 $100.88   11.400%   $112.3803   $112.38   $112.40   $112.40  
  7 purchase totals = $850.62       $920.3226   $920.30   $920.25   $920.30