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 $129.84   7.471%   $139.5403   $139.54   $139.55   $139.50  
  Purchase #2 $112.90   6.227%   $119.9303   $119.93   $119.95   $119.90  
  Purchase #3 $119.54   6.430%   $127.2264   $127.23   $127.25   $127.20  
  Purchase #4 $228.71   7.336%   $245.4882   $245.49   $245.50   $245.50  
  Purchase #5 $187.52   7.796%   $202.1391   $202.14   $202.15   $202.10  
  Purchase #6 $205.72   6.180%   $218.4335   $218.43   $218.45   $218.40  
  Purchase #7 $97.88   7.260%   $104.9861   $104.99   $105.00   $105.00  
  7 purchase totals = $1,082.11       $1,157.7439   $1,157.75   $1,157.85   $1,157.60