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 $152.63   12.240%   $171.3119   $171.31   $171.30   $171.30  
  Purchase #2 $229.93   7.281%   $246.6712   $246.67   $246.65   $246.70  
  Purchase #3 $202.64   6.135%   $215.072   $215.07   $215.05   $215.10  
  Purchase #4 $24.73   3.252%   $25.5342   $25.53   $25.55   $25.50  
  Purchase #5 $181.91   12.799%   $205.1927   $205.19   $205.20   $205.20  
  Purchase #6 $22.67   12.428%   $25.4874   $25.49   $25.50   $25.50  
  Purchase #7 $172.45   11.285%   $191.911   $191.91   $191.90   $191.90  
  7 purchase totals = $986.96       $1,081.1804   $1,081.17   $1,081.15   $1,081.20