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 $104.54   13.220%   $118.3602   $118.36   $118.35   $118.40  
  Purchase #2 $172.46   8.329%   $186.8242   $186.82   $186.80   $186.80  
  Purchase #3 $134.29   11.699%   $150.0006   $150.00   $150.00   $150.00  
  Purchase #4 $167.90   12.832%   $189.4449   $189.44   $189.45   $189.40  
  Purchase #5 $242.45   13.810%   $275.9323   $275.93   $275.95   $275.90  
  Purchase #6 $20.93   6.214%   $22.2306   $22.23   $22.25   $22.20  
  Purchase #7 $243.43   7.538%   $261.7798   $261.78   $261.80   $261.80  
  7 purchase totals = $1,086.00       $1,204.5726   $1,204.56   $1,204.60   $1,204.50