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 $48.57   11.182%   $54.0011   $54.00   $54.00   $54.00  
  Purchase #2 $131.41   10.145%   $144.7415   $144.74   $144.75   $144.70  
  Purchase #3 $241.84   12.442%   $271.9297   $271.93   $271.95   $271.90  
  Purchase #4 $111.69   6.287%   $118.712   $118.71   $118.70   $118.70  
  Purchase #5 $72.72   9.319%   $79.4968   $79.50   $79.50   $79.50  
  Purchase #6 $244.29   3.619%   $253.1309   $253.13   $253.15   $253.10  
  Purchase #7 $205.62   10.803%   $227.8331   $227.83   $227.85   $227.80  
  7 purchase totals = $1,056.14       $1,149.8451   $1,149.84   $1,149.90   $1,149.70