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 $153.92   6.386%   $163.7493   $163.75   $163.75   $163.70  
  Purchase #2 $159.51   6.624%   $170.0759   $170.08   $170.10   $170.10  
  Purchase #3 $25.94   11.445%   $28.9088   $28.91   $28.90   $28.90  
  Purchase #4 $174.54   10.251%   $192.4321   $192.43   $192.45   $192.40  
  Purchase #5 $17.40   11.722%   $19.4396   $19.44   $19.45   $19.40  
  Purchase #6 $236.81   9.833%   $260.0955   $260.10   $260.10   $260.10  
  Purchase #7 $232.84   9.112%   $254.0564   $254.06   $254.05   $254.10  
  7 purchase totals = $1,000.96       $1,088.7576   $1,088.77   $1,088.80   $1,088.70